(svn r6047) -Codechange: FOR_ALL now _only_ loops valid items, and skips invalid ones

-Codechange: use IsValidXXX where ever possible
  Note: both changes to prepare for new pool system, which needs those changes.
  For every pool there are 2 ugly lines, which will be removed when done
  implementing new pool system.
  Based on FS#13 by blathijs, partly implemented.

order.h

@@ -117,7 +117,15 @@ static inline uint16 GetOrderPoolSize(void)
 	return _order_pool.total_items;
 }
 
-#define FOR_ALL_ORDERS_FROM(order, start) for (order = GetOrder(start); order != NULL; order = (order->index + 1 < GetOrderPoolSize()) ? GetOrder(order->index + 1) : NULL)
+/**
+ * Check if a Order really exists.
+ */
+static inline bool IsValidOrder(const Order *o)
+{
+	return o->type != OT_NOTHING;
+}
+
+#define FOR_ALL_ORDERS_FROM(order, start) for (order = GetOrder(start); order != NULL; order = (order->index + 1 < GetOrderPoolSize()) ? GetOrder(order->index + 1) : NULL) if (IsValidOrder(order))
 #define FOR_ALL_ORDERS(order) FOR_ALL_ORDERS_FROM(order, 0)