Fix GH-16593: Assertion failure in DOM->replaceChild

This is already forbidden by libxml, but this condition isn't properly
checked; so the return value and lack of error makes it seem l

Fix GH-16593: Assertion failure in DOM->replaceChild

This is already forbidden by libxml, but this condition isn't properly
checked; so the return value and lack of error makes it seem like it
worked while it actually didn't. Furthermore, this can break assumptions
and assertions later on.

Closes GH-16596.

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