Auto merge of #13128 - emilio:stupidest-assertion-ever, r=SimonSapin
Auto merge of #13128 - emilio:stupidest-assertion-ever, r=SimonSapin

Fix debug assertion introduced in #13108 by me.

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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 #13125 (github issue number if applicable).

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- [x] These changes do not require tests because https://github.com/servo/servo/issues/13127

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I'm stupid, and when I did the last moving-the-code-around, I failed miserably
to double-check it in a debug build.

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