Avoid Marker-beacon spam on FDM NaN case

Check and bail out of marker beacon search if we are in NaN-land; this
should making it easier to trace the cause of the NaNs.

src/Instrumentation/marker_beacon.cxx

@@ -297,6 +297,10 @@ void FGMarkerBeacon::search()
     _time_before_search_sec = 0.5;
 
     const SGGeod pos = globals->get_aircraft_position();
+    if (!pos.isValid()) {
+        // avoid error flood when core positions goes wrong
+        return;
+    }
 
     // get closest marker beacon - within a 1nm cutoff
     BeaconFilter filter;