mon/OSDMonitor: fix base case for loading full osdmap

Right after cluster creation, first_committed is 1 and latest stashed in 0,
but we don't have the initial full map yet.  Thereafter, we do (because we
write it with trim).  Fixes afd6c7d8247075003e5be439ad59976c3d123218.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>