Auto merge of #28636 - negator:patch-3, r=jdm

Perform non-null checks on pointers used in js stacktrace captures

Need to ensure that external passed in pointers are non-null prior to using them unchecked. Seeing sporadic crashes when visiting `https:://cnn.com` and `https://www.bankofamerica.com`:

```
   ...
   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
...
```

https://github.com/rust-lang/rust/blob/d384ff79308c4eb0a3679817cd0def0147ad8612/library/core/src/ptr/non_null.rs#L181-L188

Full stacktrace:
[trace.txt](https://github.com/servo/servo/files/7631210/trace.txt)

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