Windows: buf_SetDirty do nothing if length is zero

Windows: buf_SetDirty do nothing if length is zero

Reviewed-on: http://gerrit.openafs.org/5358
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 7dcc4766377b99c60db60328c21a3990db132268)

Change-Id: I30dc0f218df56e6e38fe6ce3736a5450b7036e51
Reviewed-on: http://gerrit.openafs.org/6024
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>