We must allocate a larger ref_pointer_array. We failed to account for extra
items allocated here:
#0  find_order_in_list 
  uint el= all_fields.elements;
  all_fields.push_front(order_item); /* Add new field to field list. */
  ref_pointer_array[el]= order_item;
  order->item= ref_pointer_array + el;
#1  setup_order
#2  setup_without_group
#3  JOIN::prepare