get_packet can return null if we are terminating

src/zm_monitor.cpp

@@ -1991,6 +1991,7 @@ bool Monitor::Analyse() {
                   // Write out starting packets, do not modify packetqueue it will garbage collect itself
                   while ( start_it != snap_it ) {
                     ZMPacket *p = packetqueue->get_packet(&start_it);
+                    if ( !p ) break;
                     event->AddPacket(p);
                     p->unlock();
                     start_it ++;