Try not to explode if given IPv6 addresses

Bug report by @augmentedfourth, who actually runs real machines with
IPv6 (unlike me). This isn't exactly making DWiki IPv6 enabled as such
(that would take a version of netblock that knew about IPv6, for a
start), but at least it will hopefully no longer log warning messages or
throw exceptions if you talk to it from an IPv6 address.