Fix a problem in code for percent_rank().

The problem popped up when running win_percent_cume.test
with --ps-protocol.

sql/item_windowfunc.h

@@ -409,6 +409,7 @@ class Item_sum_percent_rank: public Item_sum_window_with_row_count
   void cleanup()
   {
     peer_tracker.cleanup();
+    Item_sum_num::cleanup();
   }
 };