Bug#57113: ha_partition::extra(ha_extra_function):
            Assertion `m_extra_cache' failed

Fix for bug#55458 included DBUG_ASSERTS causing
debug builds of the server to crash on
another multi-table update.

Removed the asserts since they where wrong.
(updated after testing the patch in 5.5).

mysql-test/r/partition.result:
  updated result
mysql-test/t/partition.test:
  Added test for bug#57113
sql/ha_partition.cc:
  Removed the assert for m_extra_cache when
  ::extra(HA_PREPARE_FOR_UPDATE) was called.