(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.

signs.h

@@ -17,6 +17,14 @@ typedef struct SignStruct {
 
 extern MemoryPool _sign_pool;
 
+/**
+ * Check if a Sign really exists.
+ */
+static inline bool IsValidSign(SignStruct* ss)
+{
+	return ss->str != 0;
+}
+
 /**
  * Get the pointer to the sign with index 'index'
  */