Windows: CM_SCACHESYNC_STOREDATA for non-files have no buffers

Windows: CM_SCACHESYNC_STOREDATA for non-files have no buffers

Do not add QData objects with null cm_buf_t pointers to the
cm_scache_t bufWritep queue when synchronizing directory changes.
If a callback is required while the directory change is being
pushed it can result in a deadlock.

LICENSE MIT

Change-Id: Id66c52ed794ceec44011167659e41a0b61898625
Reviewed-on: http://gerrit.openafs.org/1761
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>