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/backends/meta-monitor-manager.c

@@ -994,6 +994,7 @@ get_connector_type_name (MetaConnectorType connector_type)
     case META_CONNECTOR_TYPE_DSI: return "DSI";
     default: g_assert_not_reached ();
     }
+  return NULL;
 }
 
 static GList *