MDEV-21452 fixup: Avoid an unnecessary mutex operation

Author: dr-m

extra/mariabackup/xtrabackup.cc

@@ -2477,11 +2477,11 @@ xb_write_delta_metadata(const char *filename, const xb_delta_info_t *info)
 /* ================= backup ================= */
 void xtrabackup_io_throttling()
 {
- if (!xtrabackup_backup)
+ if (!xtrabackup_backup || !xtrabackup_throttle)
  return;
 
  mysql_mutex_lock(&log_sys.mutex);
- if (xtrabackup_throttle && (io_ticket--) < 0)
+ if (io_ticket-- < 0)
  mysql_cond_wait(&wait_throttle, &log_sys.mutex);
  mysql_mutex_unlock(&log_sys.mutex);
 }