Fill _res.sort_list with harmless entry. sortlist for IPv6/IPv4

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.

lib/libc/net/res_init.c

@@ -429,6 +429,9 @@ res_init()
 				    u++;
 				}
 			    }
+			    _res.sort_list[nsort].addr.s_addr =
+				(u_int32_t)0xffffffff;
+			    _res.sort_list[nsort].mask = (u_int32_t)0xffffffff;
 			    nsort++;
 			}
 			*cp = n;