On failure, Mariabackup --backup --safe-slave-backup may forget to START SLAVE SQL_THREAD

In Mariabackup, the function backup_start() that is invoked by xtrabackup_backup_func() that implements the --backup option is performing some steps that should be undone even in case of a failure. These steps are currently executed at the start of the function backup_finish() which should be invoked after a successful operation only:

	/* release all locks */

	if (!opt_no_lock) {

		unlock_all(mysql_connection);

		history_lock_time = 0;

	} else {

		history_lock_time = time(NULL) - history_lock_time;

	}

	if (opt_safe_slave_backup && sql_thread_started) {

		msg("Starting slave SQL thread\n");

		xb_mysql_query(mysql_connection,

				"START SLAVE SQL_THREAD", false);

	}

These steps, especially the START SLAVE SQL_THREAD, should be invoked also after a failed operation.