BUG#18036 - update of table joined to self reports table as crashed

Set exclude_from_table_unique_test value back to FALSE. It is needed for
further check in multi_update::prepare whether to use record cache.


sql/sql_update.cc:
  Set exclude_from_table_unique_test value back to FALSE. It is needed for
  further check in multi_update::prepare whether to use record cache.