Auto merge of servo#18302 - upsuper:atom-assert, r=bholley

Harden assert for creating atom from raw pointer

One of Stylo's common assertion turns out to be from having a null Atom (see [Gecko 1385925 bug comment 7](https://bugzilla.mozilla.org/show_bug.cgi?id=1385925#c7)). It would be helpful if we can catch this kind of crash earlier via a release-assertion.

This may regress performance to some extent which I'm not sure. But we can probably have it there for diagnosis for now, and remove later.

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Auto merge of servo#16962 - hiikezoe:prefixed-intrinsic-size-value, r…

…=Manishearth

Prefixed intrinsic size value

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1355402
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix servo#16788
- [X] These changes do not require tests because it's for stylo

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