sql/sql_select.cc:
  A fix (bug #9401: type_timestamp segfaults mysqld).
  1. create_tmp_field_from_field() is modified to take 'Item_ref *item' and 'char *name'
  instead of 'bool modify_item' and 'Item *item'.
  2. For Item_sum_min/max we should never modify item, so NULL is 
  passed as item to the create_tmp_field_from_field().