Forbid renegotiation in TLS 1.3.

Change-Id: I1b34acbbb5528e7e31595ee0cbce7618890f3955
Reviewed-on: https://boringssl-review.googlesource.com/8669
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/s3_pkt.c b/ssl/s3_pkt.c
index 1bbed59..dec8288 100644
--- a/ssl/s3_pkt.c
+++ b/ssl/s3_pkt.c
@@ -353,6 +353,10 @@
 }
 
 static int ssl3_can_renegotiate(SSL *ssl) {
+  if (ssl->server || ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
+    return 0;
+  }
+
   switch (ssl->renegotiate_mode) {
     case ssl_renegotiate_never:
       return 0;