Windows: AFSPopulateNameArrayFromRelatedArray change

Modify AFSPopulateNameArrayFromRelatedArray so that the DirectoryCB
parameter can safely be NULL.   A NULL DirectoryCB input is required
to copy the entire NameArray.

Change-Id: Id125bb22df89c93e60d485e178bc7be0ab3c313e
Reviewed-on: http://gerrit.openafs.org/9340
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>