HPCC-9127 - Fix global merge regression

Fixes for HPCC-8992, introduced a new issue.
When splitting the sample, the pending row was added
unconditionally. For the fpos' to be correct, it needed to check
if candidate to skip following a split.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4227 from jakesmith/hpcc-9127

HPCC-9127 - Fix global merge regression

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>