HPCC-10287 - Add check for inadequencies of transaction impl.

Avoid assert/crash if subfile being checked isn't actually in
the commited superfile structure, e.g. because it has just been
added in the transaction

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5073 from jakesmith/hpcc-10287b

HPCC-10287 - Add check for inadequencies of transaction impl.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>