mon/PGMonitor: fix segfault when osdmap and pgmap stats are out of sync

The leader mon is responsible for propagating changes in the osdmap
into the pgmap (e.g., when a pool or osd goes away).  If that hasn't
happened yet, the pg_map map will be out of sync with the current set
of pools.  We shouldn't crash.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #640 from ceph/wip-mon-pgmap

fix mon crash when osdmap and pgmap aren't yet synced

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>