HPCC-12000 CDfsLogicalFileName::set,removeForeign broken.

Regression introduced by HPCC-11571.
Was causing set DFLN to become blank.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6293 from jakesmith/hpcc-12000

HPCC-12000 CDfsLogicalFileName::set,removeForeign broken.

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>