o Minor bug fixes (addresses, testing):
    - Handle errors in get_interface_address6_via_udp_socket_hack by
      returning an empty list (no addresses found). This bug was triggered
      in ElectroBSD/FreeBSD jails.
    - Ensure that either a valid address is returned in address pointers,
      or that the address data is zeroed on error.
    - Ensure that free_interface_address6_list handles NULL lists.
    - Add unit tests for get_interface_address* failure cases.
      Fixes bug #17173.
      Patch by fk/teor, not in any released version of tor.