Fix heap-based overflow in parse_mqtt

PUBLISHED_FROM=3306592896298597fff5269634df0c1a1555113b
int _isatty(int fd) {
  /* 0, 1 and 2 are TTYs. */
  return fd < 2;
}

int _isatty(int fd) {
  /* 0, 1 and 2 are TTYs. */
  return fd < 2;
}
