Problem was that if copy_data_between_tables() didn't do proper
clean up in case of failures:
- copy object was not properly freed
- end_bulk_insert() was not called
- mysql_trans_prepare_alter_copy_data() set THD->transaction.on to
  false which was not properly restored

The last part caused a crash in Aria as Aria depends on that THD
is correct.

Other things:
- Reset info->switched_transactional after usage (safety)
- Reset bulk_insert_single_undo (safety)