get_interface_address6_list(): Bring back a return code check

get_interface_address6_list(): Bring back a return code check

... that was removed by 31eb486c46 which first appeared in
0.2.7.3-rc.

If tor is running in a ElectroBSD (or FreeBSD) jail it can't
get any IP addresses that aren't assigned to the jail by
looking at the interfaces and (by design) the
get_interface_address6_via_udp_socket_hack() fallback doesn't
work either.

The missing return code check resulted in tor_addr_is_internal()
complaining about a "non-IP address of type 49", due to reading
uninitialised memory.

Fixes #17173.