[Bug 727818] waylandsink: fix leak and locking mismatch on assert

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=727818
  GStreamer | gst-plugins-bad | git

Bug 727818 depends on bug 726193, which changed state.

Bug 726193 Summary: waylandsink: subsurface & scaling support, plus many other improvements
https://bugzilla.gnome.org/show_bug.cgi?id=726193

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

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