avm1: Don't attempt to serialize MovieClips

[adrian17]

This fixes the stack overflow in #4631 (after you change locations after getting the crowbar early on)

Note: if there ever was a special case in FP where it did somehow serialize some contents of MovieClips, this will break saving this.
In particular, I haven't tested custom AS classes deriving from MovieClip, as... I don't really know how :( Feel free to block this if you think this needs to be checked first.

[Herschel]

Looks like this should apply to any display object, and so only needs to check as_stage_object. Did a quick test with a class linked to a clip, and same behavior: it's skipped. MC still gets skipped if you wipe out clip.__proto__, too, so I don't think is_instance_of is the right check there.

(Probably somewhat related to #1513, where AVM1 display objects aren't really objects so much as string paths under the hood.)

[Herschel]

Thanks!