HPCC-12588 Fix a spurious assert hit during compactRows

Also make compactRows return # of freed pages instead of boolean

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6648 from jakesmith/hpcc-12588

HPCC-12588 Fix a spurious assert hit during compactRows

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>