This change required some special treatment, as HMAC is intertwined
with EVP_MD. For now, all local HMAC_CTX variables MUST be
initialised with HMAC_CTX_EMPTY, or whatever happens to be on the
stack will be mistaken for actual pointers to EVP_MD_CTX. This will
change as soon as HMAC_CTX becomes opaque.
Also, since HMAC_CTX_init() can fail now, its return type changes from
void to int, and it will return 0 on failure, 1 on success.
Reviewed-by: Rich Salz <rsalz@openssl.org>
fa0c23de83