Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

src/core/events.c

@@ -86,6 +86,7 @@ get_window_for_event (MetaDisplay        *display,
       return display->grab_window;
     default:
       g_assert_not_reached ();
+      return NULL;
     }
 }