In case of failure in ALTER ... PARTITION under LOCK TABLE
the server could crash, due to it had modified the locked
table object, which was not reverted in case of failure,
resulting in a bad table definition used after the failed
command.

Solved by always closing the LOCKED TABLE, even in case
of error.

Note: this is a 5.1-only fix, bug#56172 fixed it in 5.5+