fix mon crash when osdmap and pgmap aren't yet synced

fixes this: 2013-09-25 15:06:23.307320 7fabf13a3700 -1 *** Caught signal (Segmentation fault) **
 in thread 7fabf13a3700

 ceph version 0.69-280-g60d541d (60d541d36d3ebf3a6d8ef411a026ca31da99b95d)
 1: ceph-mon() [0x80fdea]
 2: (()+0xfcb0) [0x7fabf58b9cb0]
 3: (PGMonitor::get_health(std::list<std::pair<health_status_t, std::string>, std::allocator<std::pair<health_status_t, std::string> > >&, std::list<std::pair<health_status_t, std::string>, std::allocator<std::pair<health_status_t, std::string> > >*) const+0x1abe) [0x5fb20e]
 4: (Monitor::get_health(std::string&, ceph::buffer::list*, ceph::Formatter*)+0xcb) [0x55bb4b]
 5: (Monitor::get_status(std::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >&, ceph::Formatter*)+0x62) [0x55d102]
 6: (Monitor::handle_command(MMonCommand*)+0x19f4) [0x56d524]
 7: (Monitor::_ms_dispatch(Message*)+0xcbb) [0x574a6b]
 8: (Monitor::ms_dispatch(Message*)+0x32) [0x58c7d2]
 9: (DispatchQueue::entry()+0x549) [0x7e60b9]
 10: (DispatchQueue::DispatchThread::entry()+0xd) [0x71902d]
 11: (()+0x7e9a) [0x7fabf58b1e9a]
 12: (clone()+0x6d) [0x7fabf3eb5cbd]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to interpret this.