Add CBB_zero to set a CBB to the zero state.

One tedious thing about using CBB is that you can't safely CBB_cleanup
until CBB_init is successful, which breaks the general 'goto err' style
of cleanup. This makes it possible:

  CBB_zero ~ EVP_MD_CTX_init
  CBB_init ~ EVP_DigestInit
  CBB_cleanup ~ EVP_MD_CTX_cleanup

Change-Id: I085ecc4405715368886dc4de02285a47e7fc4c52
Reviewed-on: https://boringssl-review.googlesource.com/5267
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_asn1.c

diff --git a/ssl/ssl_asn1.c b/ssl/ssl_asn1.c
index 76052df..531e0d5 100644
--- a/ssl/ssl_asn1.c
+++ b/ssl/ssl_asn1.c
@@ -163,11 +163,9 @@
     return 0;
   }
 
-  if (!CBB_init(&cbb, 0)) {
-    return 0;
-  }
-
-  if (!CBB_add_asn1(&cbb, &session, CBS_ASN1_SEQUENCE) ||
+  CBB_zero(&cbb);
+  if (!CBB_init(&cbb, 0) ||
+      !CBB_add_asn1(&cbb, &session, CBS_ASN1_SEQUENCE) ||
       !CBB_add_asn1_uint64(&session, SSL_SESSION_ASN1_VERSION) ||
       !CBB_add_asn1_uint64(&session, in->ssl_version) ||
       !CBB_add_asn1(&session, &child, CBS_ASN1_OCTETSTRING) ||