Use OpenSSL_MAC and OpenSSL_MAC_CTX wrappers together with
OPENSSL_TRY to replace manual EVP_MAC resource management in
KMAC::sign(). This eliminates manual EVP_MAC_free/EVP_MAC_CTX_free
calls in every error path.
The code was printing one error message only, but multiple can be
generated in one call. Additionally, using this builtin produces
a much more descriptive output.
