size_t mg_mqtt_next_sub(struct mg_mqtt_message *msg, struct mg_str *topic,
                        uint8_t *qos, size_t pos) {
  uint8_t tmp;
  return mg_mqtt_next_topic(msg, topic, qos == NULL ? &tmp : qos, pos);
}
size_t mg_mqtt_next_sub(struct mg_mqtt_message *msg, struct mg_str *topic,
                        uint8_t *qos, size_t pos) {
  uint8_t tmp;
  return mg_mqtt_next_topic(msg, topic, qos == NULL ? &tmp : qos, pos);
}