Don't track icao 000000 as an aircraft

(but do still decode messages for it)

track.c

@@ -814,6 +814,11 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
         return NULL;
     }
 
+    if (mm->addr == 0) {
+        // junk address, don't track it
+        return NULL;
+    }
+
     _messageNow = mm->sysTimestampMsg;
 
     // Lookup our aircraft or create a new one