SSL_SESSION_get0_peer - get details about peer's certificate for a session
 #include <openssl/ssl.h>
 X509 *SSL_SESSION_get0_peer(SSL_SESSION *s);
SSL_SESSION_get0_peer() returns the peer certificate associated with the
  session s or NULL if no peer certificate is available. The caller
  should not free the returned value (unless X509_up_ref(3) has also been
  called).
SSL_SESSION_get0_peer() returns a pointer to the peer certificate or NULL
  if no peer certificate is available.
Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You
    may not use this file except in compliance with the License. You can obtain
    a copy in the file LICENSE in the source distribution or at
    <https://www.openssl.org/source/license.html>.