Bug#59109 : mysqlslap crashes on mysql_fetch_row after ignoring             null from mysql_store_result.
Bug#59109 : mysqlslap crashes on mysql_fetch_row after ignoring
            null from mysql_store_result.

mysqlslap segfaults at a point when it tries to fetch rows from
the result set.

Under some circumstances, mysql_store_result can return 'NULL',
even after query execution (mysql_query) succeeds, and eventually
a segfault might occur if same unchecked return value is passed
to mysql_fetch_row.

Fixed by adding a check on mysql_store_result's return value.

davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,slanterns/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,ollie314/server,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server