libceph: handle zero-length data items

rbd needs this for null copyups -- if copyup data is all zeroes, we
want to save some I/O and network bandwidth.  See rbd_obj_issue_copyup()
in the next commit.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>