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>
Community Edition 5.0.0-3 Gold

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>