Windows: buf redir queue mgmt scp can be null

Windows: buf redir queue mgmt scp can be null

buf_InsertToRedirQueue, buf_RemoveFromRedirQueue, and
buf_MoveToHeadOfRedirQueue can be passed a NULL cm_scache_t pointer.
If the pointer is NULL (as is the case during buf_Init() processing)
the scp->redirMx must not be obtained or a crash will occur.

Change-Id: I157091afd973a8b4789976c42de253831a2698a3
Reviewed-on: http://gerrit.openafs.org/7422
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>