Fix calls to SSL_get_server_tmp_key() in TLSv1.3

The macro SSL_get_server_tmp_key() returns information about the temp key
used by the server during a handshake. This was returning NULL for TLSv1.3
and causing s_client to omit this information in its connection summary.

Fixes #3081

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3114)