ceph: only set message data pointers if non-empty

Change it so we only assign outgoing data information for messages
if there is outgoing data to send.

This then allows us to add a few more (currently commented-out)
assertions.

This is related to:
    http://tracker.ceph.com/issues/4284



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>