Windows: AFSEvaluateTargetByName NULL ptr assignment

If DirEnumEntry is NULL, then on failure of AFSEvaluateTargetByName()
a NULL pointer will be dereferenced prior to function return.

Reviewed-on: http://gerrit.openafs.org/11422
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit d93b89ddb61d44b47f039ae96af6304f2d87b5e2)

Change-Id: Ia0c500b88dd805a3f97c2e195254f9321e986531
Reviewed-on: http://gerrit.openafs.org/11423
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>