MDEV-35182 crash in online_alter_end_trans with XA over vector indexes

ONLINE ALTER didn't expect XA PREPARE to fail.
Mark rollback on failed prepare with the XA_ROLLBACK_ONLY state,
detect that in ONLINE ALTER

sql/handler.cc

@@ -1543,6 +1543,7 @@ int ha_prepare(THD *thd)
       {
         if (unlikely(prepare_or_error(ht, thd, all)))
         {
+          thd->transaction->xid_state.set_rollback_only();
           ha_rollback_trans(thd, all);
           error=1;
           break;