libfetch: parse IPv4address in IPv6address properly

This fix parsing problems of IPv6 addresses which contains IPv4
addresses. One example is ::192.168.0.1.

Documents are in below:
https://datatracker.ietf.org/doc/html/rfc2373#page-22

Sponsored by: Juniper Networks, Inc.
MFC after: 1 week
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D45896