Fix a valgrind warning spotted while trying to repeat Bug#7166

"Server crashes when running out of connections"


sql/sql_class.cc:
  thd->query_id is compared with thd->warn_id in push_warning, 
  THD::THD seems to be the best place to fix it.

sql/sql_class.cc

@@ -193,6 +193,7 @@ THD::THD()
   variables.pseudo_thread_id= 0;
   one_shot_set= 0;
   file_id = 0;
+  query_id= 0;
   warn_id= 0;
   db_charset= global_system_variables.collation_database;
   bzero(ha_data, sizeof(ha_data));