Make ntile use args[0] for it's argument.

Don't hold a separate pointer as args[0] changes during fix_fields /
split_sum_func.

sql/item_windowfunc.h

@@ -429,7 +429,6 @@ class Item_sum_ntile : public Item_sum_window_with_row_count
  public:
   Item_sum_ntile(THD* thd, Item* num_quantiles_expr) :
     Item_sum_window_with_row_count(thd, num_quantiles_expr),
-    num_quantiles_expr_(num_quantiles_expr),
     current_row_count_(0) {};
 
   double val_real()
@@ -445,7 +444,7 @@ class Item_sum_ntile : public Item_sum_window_with_row_count
       return 0;
     }
 
-    longlong num_quantiles= num_quantiles_expr_->val_int();
+    longlong num_quantiles= get_num_quantiles();
 
     if (num_quantiles <= 0) {
       my_error(ER_INVALID_NTILE_ARGUMENT, MYF(0));
@@ -489,19 +488,8 @@ class Item_sum_ntile : public Item_sum_window_with_row_count
   enum Item_result result_type () const { return INT_RESULT; }
   enum_field_types field_type() const { return MYSQL_TYPE_LONGLONG; }
 
-  bool fix_fields(THD *thd, Item **ref)
-  {
-    if (Item_sum_window_with_row_count::fix_fields(thd, ref))
-      return true;
-    // TODO-cvicentiu is ref as a parameter here ok?
-    if (!num_quantiles_expr_->fixed)
-      num_quantiles_expr_->fix_fields(thd, ref);
-
-    return false;
-  }
-
  private:
-  Item* num_quantiles_expr_;
+  longlong get_num_quantiles() { return args[0]->val_int(); }
   ulong current_row_count_;
 };