openssl: Don't pre-initialize OpenSSL HMAC with an empty key

openssl: Don't pre-initialize OpenSSL HMAC with an empty key

With OpenSSL commit 929b0d70c19f60227f89fac63f22a21f21950823 setting an empty
key fails if no previous key has been set on that HMAC.

In 9138f49e we explicitly added the check we remove now, as HMAC_Update()
might crash if HMAC_Init_ex() has not been called yet. To avoid that, we
set and check a flag locally to let any get_mac() call fail if set_key() has
not yet been called.