Auto merge of #15145 - SwagColoredKitteh:issue-15101, r=emilio

Prevent crashing when a link tag has two or more in-flight requests (fix for issue #15101)

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The `HTMLLinkElement::set_stylesheet` function now checks whether there already is a stylesheet, and if there is, calls `Document::invalidate_stylesheets` after modifying `self.stylesheet`.

This PR also includes a minimal WPT that causes the panic.

This is fundamentally a timing issue, so while this fix prevents the crash, it does not fix the underlying issue. Making a &lt;link&gt; element send a second request before the first can finish and then getting the two stylesheet responses out-of-order will apply the wrong stylesheet, as demonstrated with https://gist.github.com/SwagColoredKitteh/2c24c7fac635445042eda4a30e10420e.

r? @emilio

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15101 (github issue number if applicable).

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