Windows: buf_CleanLocked protect against NULL bp->userp

Windows: buf_CleanLocked protect against NULL bp->userp

The cm_buf_t.userp field should never be NULL if the CM_BUF_DIRTY
flag is set but apparently it sometimes is.  cm_BufWrite() requires
that the userp parameter be non-NULL.  Otherwise, an assertion fails
and afsd_service.exe panics.  If bp->userp is NULL, use cm_rootUserp.
The worst that will happen is the write will fail due to an access
denied error.

Change-Id: I6cc650dcffe2b0fb50ac2ce91b74e8afbdfc40ca
Reviewed-on: http://gerrit.openafs.org/8475
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>