This fixes a crash on the attached testcase when running with the async
html parser enabled. The crash also triggers on wpt. When executing
parse ops received from the async parser, we were not considering the
case where one of these operations causes the parser to abort (in the
case above, said operation is the `<popup-info></popup-info>`).
```html
<body>
<script>
class PopupInfo extends HTMLElement {
connectedCallback() {
document.open();
}
}
customElements.define('popup-info', PopupInfo);
</script>
<popup-info></popup-info>
</body>
```
The bug is very obscure and only triggers if `document.open` is called
while there is no parser-blocking script **and** the document parser is
still active.
Testing: We don't run tests on the async parser yet
Fixes: Part of https://github.com/servo/servo/issues/37418
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
dfe7117084