Validate lookahead reservation end OK before calling FollowTrainReservation

src/pbs.h

@@ -141,6 +141,7 @@ enum FollowTrainReservationFlags {
 };
 DECLARE_ENUM_AS_BIT_SET(FollowTrainReservationFlags)
 
+bool ValidateLookAhead(const Train *v);
 PBSTileInfo FollowTrainReservation(const Train *v, Vehicle **train_on_res = nullptr, FollowTrainReservationFlags flags = FTRF_NONE);
 void ApplyAvailableFreeTunnelBridgeTiles(TrainReservationLookAhead *lookahead, int free_tiles, TileIndex tile, TileIndex end);
 void TryCreateLookAheadForTrainInTunnelBridge(Train *t);