Fix incorrect accounting of rail tunnel/bridge infrastructure counts

Regression from dual rail types change

src/tunnelbridge_cmd.cpp

@@ -2244,7 +2244,7 @@ void SubtractRoadTunnelBridgeInfrastructure(TileIndex begin, TileIndex end) {
 }
 
 static void UpdateRailTunnelBridgeInfrastructure(Company *c, TileIndex begin, TileIndex end, bool add) {
-	const uint middle_len = 2 * GetTunnelBridgeLength(begin, end) * TUNNELBRIDGE_TRACKBIT_FACTOR;
+	const uint middle_len = GetTunnelBridgeLength(begin, end) * TUNNELBRIDGE_TRACKBIT_FACTOR;
 
 	if (c != NULL) {
 		uint primary_count = middle_len + GetTunnelBridgeHeadOnlyPrimaryRailInfrastructureCount(begin) + GetTunnelBridgeHeadOnlyPrimaryRailInfrastructureCount(end);

src/tunnelbridge_map.h

@@ -223,6 +223,7 @@ static inline bool HasAcrossTunnelBridgeReservation(TileIndex t)
  */
 static inline uint GetTunnelBridgeHeadOnlyRailInfrastructureCountFromTrackBits(TrackBits bits)
 {
+	if (!bits) return 0;
 	uint pieces = CountBits(bits);
 	if (TracksOverlap(bits)) pieces *= pieces;
 	return (TUNNELBRIDGE_TRACKBIT_FACTOR / 2) * (1 + pieces);