is stored in _res_ext.sort_list, and sortlist for IPv4 is stored in
_res.sort_list for backward compatibility. However, both sort_list's
are maintaind by just one index _res.nsort. So, when IPv6 address is
specified to sortlist, empty entry was created in _res.sort_list. It
broke sortlist facility of gethostbyname().
Discussed on users@jp.ipv6.org.