Fix certificate validation for IPv6 literals in sconnect demo

Fix certificate validation for IPv6 literals in sconnect demo

Instead of naïvely trying to truncate at the first colon, use
BIO_get_conn_hostname(). That handles IPv6 literals correctly, even
stripping the [] from around them.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9201)