(svn r1834) - Fix: NPF does not check the owner of its target, busses try to enter other players' depots. TODO

- Add: asserts to find the v->u.rail.track == 0 problem.
- Add: IsValidDepot(), IsValidTown(), IsValidSign(), IsValidVehicle(), IsValidStation()
- Add: GetTileOwner(), IsTileOwner()
- Codechange: Replaced IsShipDepotTile(), IsTrainDepotTile(), IsRoadDepotTile() by IsTileDepotType().
- Codechange: typedeffed the MAP_OWNERS as Owner. Should be used as variable type.
- Codechange: Replaced a few uint by TileIndex.

vehicle.h

@@ -371,6 +371,14 @@ static inline uint16 GetVehiclePoolSize(void)
 #define FOR_ALL_VEHICLES_FROM(v, start) for (v = GetVehicle(start); v != NULL; v = (v->index + 1 < GetVehiclePoolSize()) ? GetVehicle(v->index + 1) : NULL)
 #define FOR_ALL_VEHICLES(v) FOR_ALL_VEHICLES_FROM(v, 0)
 
+/**
+ * Check if a Vehicle really exists.
+ */
+static inline bool IsValidVehicle(Vehicle* v)
+{
+	return v->type != 0;
+}
+
 /**
  * Check if an index is a vehicle-index (so between 0 and max-vehicles)
  *