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+

mysql-test/r/partition_innodb_plugin.result:
  updated result
mysql-test/t/disabled.def:
  Only disabled valgrind instead.
mysql-test/t/partition_innodb_plugin.test:
  Added test
sql/sql_partition.cc:
  close_thread_tables do not close LOCKED TABLEs
  and destroys the table object (including part_info),
  so to avoid it to be reused, always close the table
  regardless of any previous failure.