Make EVP_DigestVerifyFinal return only zero or one.

It was already almost there. Just a malloc failure away. now all the
EVP_Digest{Sign,Verify}* functions may be used without worrying about -1 return
values.

Change-Id: I96a9750b300010615979bd5f1522b1d241764665
Reviewed-on: https://boringssl-review.googlesource.com/2064
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/evp.h

diff --git a/include/openssl/evp.h b/include/openssl/evp.h
index e3922a3..1f60145 100644
--- a/include/openssl/evp.h
+++ b/include/openssl/evp.h
@@ -290,10 +290,7 @@
 
 /* EVP_DigestVerifyFinal verifies that |sig_len| bytes of |sig| are a valid
  * signature for the data that has been included by one or more calls to
- * |EVP_DigestVerifyUpdate|.
- *
- * It returns one on success and <= 0 on error. WARNING: this differs from the
- * usual return value convention. */
+ * |EVP_DigestVerifyUpdate|. It returns one on success and zero otherwise. */
 OPENSSL_EXPORT int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const uint8_t *sig,
                                          size_t sig_len);
 
@@ -462,8 +459,8 @@
  * space available at |sig|. If sufficient, the signature will be written to
  * |sig| and |*sig_len| updated with the true length.
  *
- * WARNING: Setting |out| to NULL only gives the maximum size of the
- * plaintext. The actual plaintext may be smaller.
+ * WARNING: Setting |sig| to NULL only gives the maximum size of the
+ * signature. The actual signature may be smaller.
  *
  * It returns one on success or zero on error. (Note: this differs from
  * OpenSSL, which can also return negative values to indicate an error. ) */