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.

Reviewed-on: http://gerrit.openafs.org/9903
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 975c11cb357f553f9f90a231546e0b42c6e79d31)

Change-Id: I7e90eb51193ed3b1e3305ee2127b779a79bac6b5
Reviewed-on: http://gerrit.openafs.org/9904
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>