Compute the delegated credentials length prefix with CBB.

The length prefix is trivial in this case, but using CBB means we'll
check if the length fits in a u16.

Change-Id: I7deb2348fd7784e4f7d951f56dc176df3df9ef17
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35785
Reviewed-by: Adam Langley <agl@google.com>

ssl/tls13_both.cc

diff --git a/ssl/tls13_both.cc b/ssl/tls13_both.cc
index ba5719f..bd0bb4f 100644
--- a/ssl/tls13_both.cc
+++ b/ssl/tls13_both.cc
@@ -488,10 +488,10 @@
 
   if (ssl_signing_with_dc(hs)) {
     const CRYPTO_BUFFER *raw = dc->raw.get();
+    CBB child;
     if (!CBB_add_u16(&extensions, TLSEXT_TYPE_delegated_credential) ||
-        !CBB_add_u16(&extensions, CRYPTO_BUFFER_len(raw)) ||
-        !CBB_add_bytes(&extensions,
-                       CRYPTO_BUFFER_data(raw),
+        !CBB_add_u16_length_prefixed(&extensions, &child) ||
+        !CBB_add_bytes(&child, CRYPTO_BUFFER_data(raw),
                        CRYPTO_BUFFER_len(raw)) ||
         !CBB_flush(&extensions)) {
       OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);