avm1: Bail out of swapDepths on dead clips

[Herschel]

It's possible that swapDepths could be called on a removed clip, i.e. a gotoAndStop followed by a swapDepths on a clip that is not on the new frame. This would futz up the display lists in AVM1 because currently we do not clear the parent assigned for dead clips (so that unload can work), and eventually this causes a panic. Instead, let's bail out of swapDepths if the clip has been removed. This fixes #2163 and others.

This is a somewhat temporary fix until we clean up the related issues with removed AVM1 clips (#993, #1140, #1535). References to these removed clips need to become "invalid", which would more properly solve these issues (trying to call the swapDepths method or otherwise resolve these clips would silently fail).

[Herschel]

@Toad06 Could you try out this fix on the content that was hitting this panic?

[Toad06]

I confirm these games no longer panic: #1671, #1942, #1991, #2109, #2131, #2163. Most of them work perfectly now.