Windows: AFSDeleteDirEntry Protect against NULL reference

Now that AFSDeleteDirEntry can be called with a DirectoryCB
whose ObjectInformation pointer can be NULL, protect against unintentional
dereference.

Change-Id: I6ffd21c35da5581a7ebac6987324e2ff3a8ccd2b
Reviewed-on: http://gerrit.openafs.org/9691
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>