Make sure to return the result value when calling Item_windowfunc::val_real()

This caused Item_windowfunc to return garbage.

sql/item_windowfunc.h

@@ -497,6 +497,7 @@ class Item_window_func : public Item_result_field
       res= window_func->val_real();
       null_value= window_func->null_value;
     }
+    return res;
   }
 
   longlong val_int()