update_system_hostname() was bailing out if (there is no IP4 config or
the IP4 config has no addresses) AND (there is no IP6 config or the
IP6 config has no addresses), but it would then hit an assertion and
crash if there was a valid IP6 config along with an IP4 config with no
addresses. Fix that and get rid of some redundancy.

Sort of pointed out by Coverity.