MDEV-25651 Server crash or assertion failure in THD::update_stats upon concurrent DROP TRIGGER

The problem was that in a timeout event,
thd->lex->restore_backup_query_tables_list() was called when it should
not have been.

Patch tested with the script in MDEV-25651 (not suitable for mtr)

sql/sql_trigger.cc

@@ -396,6 +396,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   bool result= TRUE;
   String stmt_query;
   bool lock_upgrade_done= FALSE;
+  bool backup_of_table_list_done= 0;;
   MDL_ticket *mdl_ticket= NULL;
   MDL_request mdl_request_for_trn;
   Query_tables_list backup;
@@ -465,6 +466,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
       Protect the query table list from the temporary and potentially
       destructive changes necessary to open the trigger's table.
     */
+    backup_of_table_list_done= 1;
     thd->lex->reset_n_backup_query_tables_list(&backup);
     /*
       Restore Query_tables_list::sql_command, which was
@@ -634,7 +636,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
     mdl_ticket->downgrade_lock(MDL_SHARED_NO_READ_WRITE);
 
   /* Restore the query table list. Used only for drop trigger. */
-  if (!create)
+  if (backup_of_table_list_done)
     thd->lex->restore_backup_query_tables_list(&backup);
 
   if (!result)