dnd: Don't try to restore to parent location if parent got destroyed

The original parent of a dragged actor might have been destroyed after the drag has been started. When the drag is canceled and _dragOrigParent is set, the code is trying to get the current position and size of it when restoring. With a destroyed parent this however would result in a crash.

This could happen for example when starting a drag on a window preview while the overview is hiding and then releasing it once the transition is done.

Fixes #4024 (closed)

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