MDEV-18875 Assertion `thd->transaction.stmt.ha_list == __null || trans == &thd->transaction.stmt' failed or bogus ER_DUP_ENTRY upon ALTER TABLE with versioning

Cause:
* when autocommit=0 (or transaction is issued by user),
 `ha_commit_trans` is called twice on ALTER TABLE, causing a duplicated
 insert into `transaction_registry` (ER_DUP_ENTRY).

Solution:
* ALTER TABLE makes an implicit commit by a second call. We actually
 need to make an insert only when it is a real commit. So is_real
 variable is additionally checked.