Val_str function added for the percentile_disc function, as it can ha…

…ve result type as STRING_RESULT

sql/item_windowfunc.h

@@ -765,6 +765,17 @@ class Item_sum_percentile_disc : public Item_sum_cume_dist,
     return value->val_decimal(dec);
   }
 
+  String* val_str(String *str)
+  {
+    if (get_row_count() == 0 || get_arg(0)->is_null())
+    {
+      null_value= true;
+      return 0;
+    }
+    null_value= false;
+    return value->val_str(str);
+  }
+
   bool add()
   {
     Item *arg = get_arg(0);