(svn r22710) -Fix [FS#4704]: Allow AIAirport::GetNoiseLevelIncrease() also for expired airports.

src/ai/api/ai_airport.cpp

@@ -131,7 +131,7 @@
 	extern uint8 GetAirportNoiseLevelForTown(const AirportSpec *as, TileIndex town_tile, TileIndex tile);
 
 	if (!::IsValidTile(tile)) return -1;
-	if (!IsValidAirportType(type)) return -1;
+	if (!IsAirportInformationAvailable(type)) return -1;
 
 	if (_settings_game.economy.station_noise_level) {
 		const AirportSpec *as = ::AirportSpec::Get(type);

src/ai/api/ai_airport.hpp

@@ -180,6 +180,7 @@ public:
 	 *  built at this tile.
 	 * @param tile The tile to check.
 	 * @param type The AirportType to check.
+	 * @pre IsAirportInformationAvailable(type).
 	 * @return The amount of noise added to the nearest town.
 	 * @note The noise will be added to the town with TownID GetNearestTown(tile, type).
 	 */