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>
3 files changed