Windows: Protect AFSDeleteObjectInfo calls

Only permit AFSDeleteObjectInfo() to be called if the reference
count returned by AFSObjectInfoDecrement() is zero.

Change-Id: I6fd65ac41467ec2aa82d4f86f0d57f91595930c1
Reviewed-on: http://gerrit.openafs.org/9671
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>