ha_innobase::prepare_inplace_alter_table(): If the ALTER TABLE
operation is no-op for InnoDB, do reset m_prebuilt->trx_id
so that ha_innobase::table_version() will always report either 0
or the identifier of the transaction that would commit changes
to the InnoDB data dictionary.

The failure scenario involved a completed DROP INDEX followed by
a no-op ALTER TABLE during which the server was killed.
An effort to create a reproducible test failed.