Thanks to Scott McPeak for reporting this and providing a patch.
* libguile/socket.c (scm_to_ipv6): When address is the wrong type,
provide more information in the exception message.
(scm_to_sockaddr): scm_to_ipv6 expects just an address, not the
whole vector.
* test-suite/tests/socket.test ("AF_INET6/SOCK_STREAM"): New set of
tests.