SSL_get_peer_cert_chain() does not work after a resumption

After a resumption it is documented that SSL_get_peer_cert_chain() will
return NULL. In BoringSSL it still returns the chain. We don't support that
so we should update the shim to call SSL_get_peer_certificate() instead
when checking whether a peer certificate is available.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2942)