wayland: Unset keyboard/pointer focus when releasing the data for these devices

Otherwise the focus_surface_listener list element becomes stale, and then
mangled if the devices' data is initialized again, and the memory memset().

https://bugzilla.gnome.org/show_bug.cgi?id=733563

src/wayland/meta-wayland-pointer.c

@@ -214,6 +214,7 @@ meta_wayland_pointer_init (MetaWaylandPointer *pointer,
 void
 meta_wayland_pointer_release (MetaWaylandPointer *pointer)
 {
+  meta_wayland_pointer_set_focus (pointer, NULL);
   set_cursor_surface (pointer, NULL);
 }