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.


client/mysqlslap.c:
  Bug#59109 : mysqlslap crashes on mysql_fetch_row after ignoring
              null from mysql_store_result.
  
  Added a check on mysql_store_result's return value. A 'NULL' return
  value here shows an erroneous situation as mysql_field_count has already
  reported a non-zero value.