We had a common pattern throughout the file to do:

  GsAppPrivate *priv = gs_app_get_instance_private (app);
  g_autoptr(GMutexLocker) locker = g_mutex_locker_new (&priv->mutex);
  g_return_if_fail (GS_IS_APP (app));

... which led to crashes when app was NULL, as g_return_if_fail was
never reached in that case. This commit reorders this so that we first
do the g_return_if_fail check and only then dereference priv.