window: Do not restore shortcuts on a NULL window

The "force restore shortcuts" being triggered by a key-combo, there is
no guarantee that the currently focused window is actually non-NULL in
which case we would crash.

Make sure there is a window currently focused before trying to restore
the shortcuts on that window.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/464

src/core/keybindings.c

@@ -3546,6 +3546,9 @@ handle_restore_shortcuts (MetaDisplay     *display,
 {
   ClutterInputDevice *source;
 
+  if (!display->focus_window)
+    return;
+
   source = clutter_event_get_source_device ((ClutterEvent *) event);
 
   meta_topic (META_DEBUG_KEYBINDINGS, "Restoring normal keyboard shortcuts\n");