Potential out of bounds access in ReplicaSetMonitor

massert( 15899, str::stream() << "No suitable secondary found for slaveOk query in replica set: "

         << _name, _master >= 0 && _nodes[_master].ok ); // <----------- _master can be out of bounds here

// Fall back to primary

verify( static_cast<unsigned>(_master) < _nodes.size() );

probablePrimaryIdx = _find_inlock( maybePrimary );

probablePrimaryConn = _nodes[probablePrimaryIdx].conn;