Make TrafficRecord own its AIAircraft.

- use the ref-counting system, hopefully avoid a dangling pointer
  crash I encountered. (Of course it may turn into a leak)

src/ATC/atc_mgr.cxx

@@ -195,8 +195,7 @@ void FGATCManager::init() {
 
 void FGATCManager::shutdown()
 {
-    delete ai_ac;
-    ai_ac = NULL;
+    ai_ac.clear();
     activeStations.clear();
 }
 

src/ATC/atc_mgr.hxx

@@ -46,7 +46,7 @@ class FGATCManager : public SGSubsystem
 {
 private:
   AtcVec activeStations;
-  FGAIAircraft* ai_ac;
+  SGSharedPtr<FGAIAircraft> ai_ac;
   FGATCController *controller, *prevController; // The ATC controller that is responsible for the user's aircraft. 
   bool networkVisible;
   bool initSucceeded;

src/ATC/trafficcontrol.cxx

@@ -180,8 +180,11 @@ FGTrafficRecord::FGTrafficRecord():
         allowPushback(true),
         priority(0),
         timer(0),
-        latitude(0), longitude(0), heading(0), speed(0), altitude(0), radius(0),
-        aircraft(NULL)
+        latitude(0), longitude(0), heading(0), speed(0), altitude(0), radius(0)
+{
+}
+
+FGTrafficRecord::~FGTrafficRecord()
 {
 }
 
@@ -208,6 +211,17 @@ void FGTrafficRecord::setPositionAndIntentions(int pos,
         }
     }
 }
+
+void FGTrafficRecord::setAircraft(FGAIAircraft *ref)
+{
+    aircraft = ref;
+}
+
+FGAIAircraft* FGTrafficRecord::getAircraft() const
+{
+    return aircraft.ptr();
+}
+
 /**
  * Check if another aircraft is ahead of the current one, and on the same
  * return true / false is the is/isn't the case.

src/ATC/trafficcontrol.hxx

@@ -161,12 +161,12 @@ private:
     FGATCInstruction instruction;
     double latitude, longitude, heading, speed, altitude, radius;
     std::string runway;
-    //FGAISchedule *trafficRef;
-    FGAIAircraft *aircraft;
+    SGSharedPtr<FGAIAircraft> aircraft;
 
 
 public:
     FGTrafficRecord();
+    virtual ~FGTrafficRecord();
 
     void setId(int val)  {
         id = val;
@@ -266,17 +266,15 @@ public:
         return runway;
     };
     //void setCallSign(string clsgn) { callsign = clsgn; };
-    void setAircraft(FGAIAircraft *ref) {
-        aircraft = ref;
-    };
+    void setAircraft(FGAIAircraft *ref);
+
     void updateState() {
         state++;
         allowTransmission=true;
     };
     //string getCallSign() { return callsign; };
-    FGAIAircraft *getAircraft() const {
-        return aircraft;
-    };
+    FGAIAircraft *getAircraft() const;
+
     int getTime() const {
         return timer;
     };