Implement certificate_authorities extension

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2918)
diff --git a/ssl/statem/statem_clnt.c b/ssl/statem/statem_clnt.c
index 4f547ac..d584bd7 100644
--- a/ssl/statem/statem_clnt.c
+++ b/ssl/statem/statem_clnt.c
@@ -2351,7 +2351,7 @@
         }
 
         if (!PACKET_get_length_prefixed_2(pkt, &extensions)) {
-                SSLerr(SSL_F_TLS_PROCESS_CLIENT_CERTIFICATE, SSL_R_BAD_LENGTH);
+                SSLerr(SSL_F_TLS_PROCESS_CERTIFICATE_REQUEST, SSL_R_BAD_LENGTH);
                 goto err;
         }
         if (!tls_collect_extensions(s, &extensions,