Windows: AFSInitFcb Check ObjectInfo->Fcb for NULL

Windows: AFSInitFcb Check ObjectInfo->Fcb for NULL

Now that AFSInitFcb is called under the ObjectInfoLock, it is
once again safe to perform a test for ObjectInfo->Fcb != NULL
and return immediately if an Fcb is already assigned.

Change-Id: Id926b9ccf24c7761dbeacf42c53d7cc7d1375482
Reviewed-on: http://gerrit.openafs.org/8226
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>