Don't overwrite ip_hton af/prefix in fr_pton4

Don't overwrite address family and prefix set by ip_hton (which can fall
back onto IPv6) with AF_INT/32, in fr_pton4.

This fixes radiusd listening on IPv4 address consiting of the first 4
octets of resolved IPv6 address when falling back to IPv6.
Merge pull request #862 from spbnick/keep_ip_hton_af_prefix

Don't overwrite ip_hton af/prefix in fr_pton4