ginetaddress: fix addr/string conversions on windows

When parsing an address, we need to re-set "len" between IPv4 and
IPv6, since WSAStringToAddress() might set it to sizeof(struct sin_addr)
when trying to parse the string as IPv4, even if it fails. Also, we
need to make sure to not pass strings to WSAStringToAddress() that it
will accept but that we don't want it to.

When stringifying an address, we need to clear the sockaddr before
filling it in, so we don't accidentally end up with an unwanted
scope_id or the like.

https://bugzilla.gnome.org/show_bug.cgi?id=701401