HPCC-12361 Assert error (hashtable not empty) from new statistics

Missing a releaseAll(), it seems.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6512 from richardkchapman/roxie-child-stats

HPCC-12361 - Assert error (hashtable not empty) from new statistics

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>