HPCC-10784 Keyed limits on stepped indexreads can be miscalculated

Fix errors in thor following the Roxie fix in this area.
    
Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

HPCC-10784 - Fix minor issues spotted in review
Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>