Windows: AFSEvaluateTargetByName free buffer if no return

Windows: AFSEvaluateTargetByName free buffer if no return

For consistency with other functions in AFSCommSupport
modify AFSEvaluateTargetByName to free the DirEntry on
completion if the caller has not provided an out parameter
to accept it.

Change-Id: Ic5b6cc655fd47b95493e6aadb7b23481d0187d5c
Reviewed-on: http://gerrit.openafs.org/5624
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>