HPCC-10896 - Lookup Join leaking rows in hash table.

The overloaded reset() of the std. lookup was resetting the
hash table size, before the constructor had a chance to free
the rows within the HT, therefore they leaked.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5433 from jakesmith/hpcc-10896

HPCC-10896 - Lookup Join leaking rows in hash table.

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>