src: don't check failure with ERR_peek_error()

It's possible there is already an existing error on OpenSSL's error
stack that is unrelated to the EVP_DigestInit_ex() operation we just
executed.

Fixes: https://github.com/nodejs/node/issues/4221
PR-URL: https://github.com/nodejs/node/pull/4731


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>