wayland: Check for NULL surface on pointer leave events
wayland: Check for NULL surface on pointer leave events

In the protocol this is the expected behaviour when the client has
destroyed the surface.

https://bugzilla.gnome.org/show_bug.cgi?id=707377

Distrotech/clutter,jigpu/clutter,Distrotech/clutter,ebassi/clutter,ebassi/clutter,ebassi/clutter,djdeath/clutter,ebassi/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,ebassi/clutter,GNOME/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,djdeath/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,jigpu/clutter,jigpu/clutter