infinite loop in mariabackup if log LOG_HEADER_FORMAT field is 0

        if (err != DB_SUCCESS) {

                msg("Error: cannot read redo log header");

unlock_and_fail:

                mysql_mutex_unlock(&log_sys.mutex);

        }

        if (log_sys.log.format == 0) {

                msg("Error: cannot process redo log before MariaDB 10.2.2");

                goto unlock_and_fail;

        }