size_t mg_mqtt_next_unsub(struct mg_mqtt_message *msg, struct mg_str *topic,
                          size_t pos) {
  return mg_mqtt_next_topic(msg, topic, NULL, pos);
}
size_t mg_mqtt_next_unsub(struct mg_mqtt_message *msg, struct mg_str *topic,
                          size_t pos) {
  return mg_mqtt_next_topic(msg, topic, NULL, pos);
}