os: don't crash on no-journal case
os: don't crash on no-journal case

JournalingObjectStore::commit_start should handle the case where journal is
null. This will occur if the user doesn't configure a journal.

Signed-off-by: Colin McCabe <c0eabe2887ac1e91c280795b9fca010e21066283@hq.newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph