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>

include/openssl/bytestring.h

diff --git a/include/openssl/bytestring.h b/include/openssl/bytestring.h
index 8583c4e..3419275 100644
--- a/include/openssl/bytestring.h
+++ b/include/openssl/bytestring.h
@@ -248,6 +248,12 @@
   char is_top_level;
 };
 
+/* CBB_zero sets an uninitialised |cbb| to the zero state. It must be
+ * initialised with |CBB_init| or |CBB_init_fixed| before use, but it is safe to
+ * call |CBB_cleanup| without a successful |CBB_init|. This may be used for more
+ * uniform cleanup of a |CBB|. */
+OPENSSL_EXPORT void CBB_zero(CBB *cbb);
+
 /* CBB_init initialises |cbb| with |initial_capacity|. Since a |CBB| grows as
  * needed, the |initial_capacity| is just a hint. It returns one on success or
  * zero on error. */