Windows: NotifyHardLink avoid null ptr reference

In AFSNotifyHardLink, if the TargetDirectoryCB out parameter is non-NULL
but pDirNode is NULL, do not attempt to increment the
DirOpenReferenceCount because doing so will trigger an exception.

Change-Id: I9d50e05aff9a4922113f814dd3d50e115725ecca
Reviewed-on: http://gerrit.openafs.org/9903
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>