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.

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>
(cherry picked from commit dcae72c87e63b595c4e86cb199ff75332b51506a)

Change-Id: Id1e4d37b055a289847d9ea2def23a402644c7080
Reviewed-on: http://gerrit.openafs.org/9896
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>