Fix refit in station with articulated vehicles with no capacity in leading part

src/articulated_vehicles.cpp

@@ -305,6 +305,19 @@ bool IsArticulatedVehicleCarryingDifferentCargoes(const Vehicle *v, CargoID *car
 	return false;
 }
 
+/**
+ * Returns the overall cargo of an articulated vehicle if all parts are refitted to the same cargo.
+ * Note: Vehicles not carrying anything are ignored
+ * @param v the first vehicle in the chain
+ * @return the common CargoID. (CT_INVALID if no part is carrying something or they are carrying different things)
+ */
+CargoID GetOverallCargoOfArticulatedVehicle(const Vehicle *v)
+{
+	CargoID cargo_id;
+	IsArticulatedVehicleCarryingDifferentCargoes(v, &cargo_id);
+	return cargo_id;
+}
+
 /**
  * Checks whether the specs of freshly build articulated vehicles are consistent with the information specified in the purchase list.
  * Only essential information is checked to leave room for magic tricks/workarounds to grfcoders.

src/articulated_vehicles.h

@@ -22,6 +22,7 @@ void GetArticulatedRefitMasks(EngineID engine, bool include_initial_cargo_type,
 CargoTypes GetUnionOfArticulatedRefitMasks(EngineID engine, bool include_initial_cargo_type);
 CargoTypes GetIntersectionOfArticulatedRefitMasks(EngineID engine, bool include_initial_cargo_type);
 bool IsArticulatedVehicleCarryingDifferentCargoes(const Vehicle *v, CargoID *cargo_type);
+CargoID GetOverallCargoOfArticulatedVehicle(const Vehicle *v);
 bool IsArticulatedVehicleRefittable(EngineID engine);
 bool IsArticulatedEngine(EngineID engine_type);
 void CheckConsistencyOfArticulatedVehicle(const Vehicle *v);

src/economy.cpp

@@ -1762,7 +1762,7 @@ static void HandleStationRefit(Vehicle *v, CargoArray &consist_capleft, Station
 	}
 
 	/* Refit if given a valid cargo. */
-	if (new_cid < NUM_CARGO && new_cid != v_start->cargo_type) {
+	if (new_cid < NUM_CARGO && new_cid != GetOverallCargoOfArticulatedVehicle(v_start)) {
 		/* INVALID_STATION because in the DT_MANUAL case that's correct and in the DT_(A)SYMMETRIC
 		 * cases the next hop of the vehicle doesn't really tell us anything if the cargo had been
 		 * "via any station" before reserving. We rather produce some more "any station" cargo than