Perform non-null checks on pointers in subsumes

Need to ensure the passed in pointers are non-null prior to using them unchecked. Seeing sporadic crashes:

```
Stack trace for thread "Script(2,36)"
   0: backtrace::backtrace::libunwind::trace
             at /Users/navgattu/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.63/src/backtrace/libunwind.rs:93:5
      backtrace::backtrace::trace_unsynchronized
             at /Users/navgattu/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.63/src/backtrace/mod.rs:66:5
   1: <servo::backtrace::Print as core::fmt::Debug>::fmt
             at /Users/navgattu/Documents/dev-git/servo/ports/winit/backtrace.rs:53:13
   2: core::fmt::write
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/core/src/fmt/mod.rs:1117:17
   3: std::io::Write::write_fmt
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/std/src/io/mod.rs:1667:15
   4: servo::backtrace::print
             at /Users/navgattu/Documents/dev-git/servo/ports/winit/backtrace.rs:17:5
   5: servo::crash_handler::install::handler
             at /Users/navgattu/Documents/dev-git/servo/ports/winit/crash_handler.rs:25:21
   6: __sigtramp
   7: core::ptr::non_null::NonNull<T>::as_ref
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/core/src/ptr/non_null.rs:317:20
   8: alloc::rc::Rc<T>::inner
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/alloc/src/rc.rs:332:18
      <alloc::rc::Rc<T> as core::clone::Clone>::clone
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/alloc/src/rc.rs:1479:9
   9: <servo_url::origin::MutableOrigin as core::clone::Clone>::clone
             at /Users/navgattu/Documents/dev-git/servo/components/url/origin.rs:92:26
  10: script::dom::bindings::principals::ServoJSPrincipals::origin
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/bindings/principals.rs:42:9
  11: script::dom::bindings::principals::subsumes
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/bindings/principals.rs:136:22
  12: _ZN2jsL30SavedFrameSubsumedByPrincipalsEP9JSContextP12JSPrincipalsN2JS6HandleIPNS_10SavedFrameEEE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/SavedStacks.cpp:617:10
      _ZN2jsL20GetFirstMatchedFrameIFbP9JSContextP12JSPrincipalsN2JS6HandleIPNS_10SavedFrameEEEEEES8_S2_S4_RT_S9_NS5_20SavedFrameSelfHostedERb
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/SavedStacks.cpp:636:9
      _ZN2jsL21GetFirstSubsumedFrameEP9JSContextP12JSPrincipalsN2JS6HandleIPNS_10SavedFrameEEENS4_20SavedFrameSelfHostedERb
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/SavedStacks.cpp:660:10
  13: _ZN2js16UnwrapSavedFrameEP9JSContextP12JSPrincipalsN2JS6HandleIP8JSObjectEENS4_20SavedFrameSelfHostedERb
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/SavedStacks.cpp:744:10
  14: _ZN2JS16BuildStackStringEP9JSContextP12JSPrincipalsNS_6HandleIP8JSObjectEENS_13MutableHandleIP8JSStringEEmN2js11StackFormatE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/SavedStacks.cpp:1041:13
  15: mozjs::rust::CapturedJSStack::as_string
             at /Users/navgattu/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/09edacd/src/rust.rs:1377:17
  16: script::dom::webglrenderingcontext::capture_webgl_backtrace::{{closure}}
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:1939:46
  17: core::option::Option<T>::and_then
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/core/src/option.rs:1043:24
  18: script::dom::webglrenderingcontext::capture_webgl_backtrace
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:1939:27
  19: script::dom::webglrenderingcontext::WebGLRenderingContext::send_command
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:397:28
  20: script::dom::webglrenderingcontext::WebGLRenderingContext::get_gl_extensions
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:913:9
  21: <script::dom::webglrenderingcontext::WebGLRenderingContext as script::dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextBinding::WebGLRenderingContextMethods>::GetSupportedExtensions::{{closure}}
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:2330:27
  22: script::dom::webgl_extensions::extensions::WebGLExtensions::init_once
  23: <script::dom::webglrenderingcontext::WebGLRenderingContext as script::dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextBinding::WebGLRenderingContextMethods>::GetSupportedExtensions
             at /Users/navgattu/Documents/dev-git/servo/components/script/dom/webglrenderingcontext.rs:2329:9
  24: script::dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextBinding::getSupportedExtensions::{{closure}}::{{closure}}
             at /Users/navgattu/Documents/dev-git/servo/target/debug/build/script-80d43fa6e481c605/out/Bindings/WebGLRenderingContextBinding.rs:1095:46
  25: script::dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextBinding::getSupportedExtensions::{{closure}}
             at /Users/navgattu/Documents/dev-git/servo/target/debug/build/script-80d43fa6e481c605/out/Bindings/WebGLRenderingContextBinding.rs:1090:33
  26: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/core/src/ops/function.rs:280:13
  27: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/core/src/panic/unwind_safe.rs:271:9
  28: std::panicking::try::do_call
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/std/src/panicking.rs:403:40
  29: <unknown>
             at /Users/navgattu/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/09edacd/src/glue.rs:299:6
  30: std::panicking::try
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/std/src/panicking.rs:367:19
  31: std::panic::catch_unwind
             at /rustc/0fa3190394475a84360b34e074e719d519bc40f1/library/std/src/panic.rs:129:14
  32: mozjs::panic::wrap_panic
             at /Users/navgattu/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/09edacd/src/panic.rs:22:11
  33: script::dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextBinding::getSupportedExtensions
             at /Users/navgattu/Documents/dev-git/servo/target/debug/build/script-80d43fa6e481c605/out/Bindings/WebGLRenderingContextBinding.rs:1090:5
  34: CallJitMethodOp
             at /Users/navgattu/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/09edacd/src/jsglue.cpp:663:12
  35: script::dom::bindings::utils::generic_call
  36: script::dom::bindings::utils::generic_method
  37: _Z12CallJSNativeP9JSContextPFbS0_jPN2JS5ValueEEN2js10CallReasonERKNS1_8CallArgsE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:435:13
      _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructENS_10CallReasonE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:520:12
  38: _ZN2js13CallFromStackEP9JSContextRKN2JS8CallArgsE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:584:10
      _ZL9InterpretP9JSContextRN2js8RunStateE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:3244:16
  39: _ZN2js9RunScriptEP9JSContextRNS_8RunStateE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:405:13
  40: _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructENS_10CallReasonE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:552:13
  41: _ZN2js4CallEP9JSContextN2JS6HandleINS2_5ValueEEES5_RKNS_13AnyInvokeArgsENS2_13MutableHandleIS4_EENS_10CallReasonE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:597:8
  42: _ZN2js9fun_applyEP9JSContextjPN2JS5ValueE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/JSFunction.cpp:1166:10
  43: _Z12CallJSNativeP9JSContextPFbS0_jPN2JS5ValueEEN2js10CallReasonERKNS1_8CallArgsE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:435:13
      _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructENS_10CallReasonE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/vm/Interpreter.cpp:520:12
  44: _ZN2js3jit14DoCallFallbackEP9JSContextPNS0_13BaselineFrameEPNS0_15ICCall_FallbackEjPN2JS5ValueENS7_13MutableHandleIS8_EE
             at /Users/navgattu/Documents/dev-git/mozjs/mozjs/js/src/jit/BaselineIC.cpp:1841:10
```

0 files changed, 0 insertions, 0 deletions