The old request code simply ignored zero-length requests. We should
still operate that same way to avoid any changes in behavior. We
can implement handling for special zero-length requests separately
(see http://tracker.ceph.com/issues/4236).
Add some assertions based on this new constraint.
This resolves:
http://tracker.ceph.com/issues/4237
Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>