(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.

town.h

@@ -133,6 +133,14 @@ VARDEF uint16 *_town_sort;
 
 extern MemoryPool _town_pool;
 
+/**
+ * Check if a Town really exists.
+ */
+static inline bool IsValidTown(Town* town)
+{
+	return town->xy != 0; /* XXX: Replace by INVALID_TILE someday */
+}
+
 /**
  * Get the pointer to the town with index 'index'
  */