HPCC-8547 - Interleaved mixed width subfiles exposing mapSubPart bug

mapSubPart was testing the wrong subfile when iterating through
parts/subs to determining which subfile and part the logical part index
referred to.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #3849 from jakesmith/mapSubPart

HPCC-8547 - Interleaved mixed width subfiles exposing mapSubPart bug

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>