HPCC-9555 - Fix possible crash when hashdedup spills

If hash dedup spills and 1st round used a higher number of buckets
than subsequent rounds, then the excess hash tables are cleared
causing the parent bucket handler to crash when accessing a
released HT

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4554 from jakesmith/hpcc-9555

HPCC-9555 - Fix possible crash when hashdedup spills

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>