rollback_inplace_alter_table(): Tolerate a case where the transaction
is not in an active state. If ha_innobase::commit_inplace_alter_table()
failed with a deadlock, the transaction would already have been
rolled back. This omission of error handling was introduced in
commit 1bd681c8 (MDEV-25506 part 3).

After commit c3c53926 (MDEV-26554)
it became easier to trigger DB_DEADLOCK during exclusive table lock
acquisition in ha_innobase::commit_inplace_alter_table().

lock_table_low(): Add DBUG injection "innodb_table_deadlock".