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

Fix setting img src before window.onload causing panic

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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.

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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 #17319.

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- [x] There are tests for these changes

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components/script/dom/htmlimageelement.rs

@@ -493,6 +493,7 @@ impl HTMLImageElement {
         request.image = None;
         request.metadata = None;
         let document = document_from_node(self);
+        LoadBlocker::terminate(&mut request.blocker);
         request.blocker = Some(LoadBlocker::new(&*document, LoadType::Image(url.clone())));
     }