osdc/Objecter: e shouldn't be zero in Objecter::handle_osd_map

Signed-off-by: Ding Dinghua <dingdinghua85@gmail.com>
osdc/Objecter: Fix a bug of dead looping in Objecter::handle_osd_map

If current map epoch is less than oldest epoch, current map epoch
should step up to oldest epoch.

Signed-off-by: Ding Dinghua <dingdinghua85@gmail.com>