assert fails for subsurface with destroyed parent

An assert is triggered when a client interacts with a subsurface after it's parent is destroyed.

Before this MR was merged gtkwaylandsink would trigger this assert (assert(view->geometry.parent)).

Weston crashes at the start of the third video when running:

gst-play-1.0 test.mp4 test.mp4 test.mp4 --videosink="gtkwaylandsink"