Windows: Protect against DirEntry with NULL ObjInfo

During cleanup protect AFSExamineObjectInfo() and AFSExamineDirectory()
from DirectoryCB objects that have a NULL ObjectInformation pointer.

Change-Id: Id46f6b53ec4861f5ac2d28b918d073201d2433ce
Reviewed-on: http://gerrit.openafs.org/9603
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>