(svn r26130) -Codechange: add some guards against using invalid image indices

src/vehicle_func.h

@@ -29,6 +29,15 @@
 static const int VEHICLE_PROFIT_MIN_AGE = DAYS_IN_YEAR * 2; ///< Only vehicles older than this have a meaningful profit.
 static const Money VEHICLE_PROFIT_THRESHOLD = 10000;        ///< Threshold for a vehicle to be considered making good profit.
 
+/**
+ * Helper to check whether an image index is valid for a particular vehicle.
+ * @param <T> The type of vehicle.
+ * @param image_index The image index to check.
+ * @return True iff the image index is valid.
+ */
+template <VehicleType T>
+bool IsValidImageIndex(uint8 image_index);
+
 typedef Vehicle *VehicleFromPosProc(Vehicle *v, void *data);
 
 void VehicleServiceInDepot(Vehicle *v);