Bug #31517: Potential crash due to access of NULL thd in mark_transaction_to_rollback()

Introduced in mark_transaction_to_rollback(), part of fix for bug 24989;
fix is to check thd for NULL before using it.

sql/sql_class.cc

@@ -2241,8 +2241,11 @@ void THD::restore_sub_statement_state(Sub_statement_state *backup)
 
 void mark_transaction_to_rollback(THD *thd, bool all)
 {
+  if (thd)
+  {
     thd->is_fatal_sub_stmt_error= TRUE;
     thd->transaction_rollback_request= all;
+  }
 }
 /***************************************************************************
   Handling of XA id cacheing