Avoid iterating vehicle list to release disaster vehicles if there are none

src/disaster_vehicle.h

@@ -45,7 +45,7 @@ struct DisasterVehicle FINAL : public SpecializedVehicle<DisasterVehicle, VEH_DI
 	DisasterVehicle();
 	DisasterVehicle(int x, int y, Direction direction, DisasterSubType subtype, VehicleID big_ufo_destroyer_target = VEH_INVALID);
 	/** We want to 'destruct' the right class. */
-	virtual ~DisasterVehicle() {}
+	virtual ~DisasterVehicle();
 
 	void UpdatePosition(int x, int y, int z);
 	void UpdateDeltaXY();