CVE-2021-22945: UAF and double-free in MQTT sending

Vulnerability Description libcurl version 7.77.0 has a [Use-After-Free](https://github.com/curl/curl/blob/curl-7_77_0/lib/mqtt.c#L559) and a [Double-Free](https://github.com/curl/curl/blob/curl-7_77_0/lib/mqtt.c#L560) in `lib/mqtt.c` in the function `mqtt_doing` on [lines 556 - …

if(mq->nsend) {
  /* send the remainder of an outgoing packet */
  char *ptr = mq->sendleftovers;
  result = mqtt_send(data, mq->sendleftovers, mq->nsend);
  free(ptr);
  if(result)
    return result;
}