mcproxy: fix mld filtering

If no source address was present, mcproxy was assuming that source address was of type IPv4, by using 0.0.0.0 as the default address. This did not work when group address was of type IPv6, because then mcproxy would compare :: with 0.0.0.0 while looping over filters, and thus filtering for IPv6 addresses would fail, so we use :: as default if group address family is not AF_INET.