Fix heap-based overflow in parse_mqtt

PUBLISHED_FROM=3306592896298597fff5269634df0c1a1555113b
MG_INTERNAL void altbuf_init(struct altbuf *ab, char *buf, size_t buf_size) {
  mbuf_init(&ab->m, 0);
  ab->user_buf = buf;
  ab->user_buf_size = buf_size;
  ab->len = 0;
}

MG_INTERNAL void altbuf_init(struct altbuf *ab, char *buf, size_t buf_size) {
  mbuf_init(&ab->m, 0);
  ab->user_buf = buf;
  ab->user_buf_size = buf_size;
  ab->len = 0;
}
