Windows: AFSClose Set FsContext* to NULL

After the Fcb OpenReferenceCount is decremented it is no longer
safe for the Fcb pointer to be referenced.  Set FsContext to NULL.

After the AFSRemoveCcb() call, it is no longer safe to reference
the Ccb pointer.  Set FsContext2 to NULL.

Change-Id: I5be1367c599222df02269bf1ed85ec89ac5d279c
Reviewed-on: http://gerrit.openafs.org/9890
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>