Set trx->mysql_query_str to NULL at transaction commit.

It could be a problem if someone looks at it after that because MySQL may
have free()d it then.

Approved by:	Heikki (via IM)

trx/trx0trx.c

@@ -933,6 +933,7 @@ trx_commit_off_kernel(
 	trx->rseg = NULL;
 	trx->undo_no = ut_dulint_zero;
 	trx->last_sql_stat_start.least_undo_no = ut_dulint_zero;
+	trx->mysql_query_str = NULL;
 
 	ut_ad(UT_LIST_GET_LEN(trx->wait_thrs) == 0);
 	ut_ad(UT_LIST_GET_LEN(trx->trx_locks) == 0);