Handle non-existent front interface in maps from older MONs

Fix OSDService::get_con_osd_hb() to not try to get_connection() without front interface
Fix OSD::handle_osd_map() to check for missing front interface

Fixes: #5460

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>