Windows: AFSEvaluateTargetByName NULL ptr assignment

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

Change-Id: I6d0d646e667c4b805abcddce1af49b81037ae51d
Reviewed-on: http://gerrit.openafs.org/11422
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>