MDEV-18078 Assertion `trnman_has_locked_tables(trn) > 0' failed

Problem was that in case of implicit rollback for alter table
Aria did try to run commit twice.

The test case for this is tricky to do in 10.2, so it will
be added to 10.4 as part of BACKUP STAGE testing.

storage/maria/ha_maria.cc

@@ -2743,7 +2743,8 @@ int ha_maria::external_lock(THD *thd, int lock_type)
     }
     else
     {
-      TRN *trn= (file->trn != &dummy_transaction_object ? file->trn : 0);
+      /* We have to test for THD_TRN to protect against implicit commits */
+      TRN *trn= (file->trn != &dummy_transaction_object && THD_TRN ? file->trn : 0);
       /* End of transaction */
 
       /*