TLS session resumption client cert bypass with CURLOPT_SSL_CTX_FUNCTION

Cipher, by Causal Security, was used to find, exploit and confirm this issue. A further check was carried out to establish, as far as is possible from outside the project, that it is neither a duplicate of an existing report or advisory nor documented as intended behaviour.

static CURLcode ssl_peer_key_build(struct ssl_primary_config *ssl,
                                   const struct ssl_peer *peer,
                                   const char *tls_id,
                                   char **ppeer_key)