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-keyboard.c

@@ -301,6 +301,7 @@ meta_wayland_xkb_info_destroy (MetaWaylandXkbInfo *xkb_info)
 void
 meta_wayland_keyboard_release (MetaWaylandKeyboard *keyboard)
 {
+  meta_wayland_keyboard_set_focus (keyboard, NULL);
   meta_wayland_xkb_info_destroy (&keyboard->xkb_info);
   xkb_context_unref (keyboard->xkb_context);