Auto merge of #17347 - duckinator:fix-issue-17319, r=mbrubeck

Fix setting img src before window.onload causing panic

<!-- Please describe your changes on the following line: -->

Setting an image source asynchronously (e.g. using `setTimeout` or in the callback of an `XMLHttpRequest`) before `window.onload` fired would cause a panic due to a preexisting load blocker not being terminated before assigning a new one.

This PR fixes that and adds a test for it.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #17319.

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17347)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions