MOSDMap: do not leave {oldest,newest}_map uninitialized when decoding old messages
MOSDMap: do not leave {oldest,newest}_map uninitialized when decoding old messages

This leads to badness like

  osd_map(295..296 src has 74308224..0) v1

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

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