Bug#19553099 EXPIRE_LOGS_DAYS=1 CRASH MYSQLD DURING RESTART, DBUG_SYNC(NULL, ...)
Bug#19553099 EXPIRE_LOGS_DAYS=1 CRASH MYSQLD DURING RESTART, DBUG_SYNC(NULL, ...)

 - Restarting mysqld with --expire-log-days=1 triggers 'log_in_use()' to be called while current_thd is NULL.
 - Check current_thd before calling DEBUG_SYNC() to avoid passing NULL pointer to DEBUG_SYNC()
 - Wrap debug code construct inside #ifndef DBUG_OFF like in other parts of the file
ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,slanterns/server,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,ollie314/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,ollie314/server