avm1: Incorrect path when accessing _root inside a button after removal

[Herschel]

Place a movieclip inside a button with this code on the movieclip's frame 1:

tellTarget(_root) {
	gotoAndStop(2); // The button no longer exists on frame 2, so the button and child movieclip are removed
}
trace(_root); // Should trace _level0, but traces _level1 in Ruffle

Sample SWF:
incorrect-level.zip

MovieClipReference::process_swf5_references can no longer access _root and ends up returning None, and DisplayObject::path ends up incorrectly returning _level1 based on the depth of the movieclip. This probably affects other properties such as _parent as well.

There should probably be an assert that we're a root movie when we do "_level" + self.depth(), because non-root movieclips should never run that code. Also, it's probably worthwhile to investigate how this interacts exactly with loadMovieNum; if you load the above movie into _level20, does trace(_root); still output _level20 in the FP?

Related to #9885.