Windows: AFSLocateNameEntry OutVolumeCB can be NULL

It is possible for the AFSLocateNameEntry OutVolumeCB parameter
to be assigned a NULL value upon return.   Handle it in the callers.

Change-Id: I15e4581d4655dbaca7c4ca4b9e9af758e97c5c95
Reviewed-on: http://gerrit.openafs.org/9464
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>