Fix EVP_CIPHER_CTX_rand_key()

Make sure we use the the correct key length in EVP_CIPHER_CTX_rand_key().
Now that ciphers may come from providers we need to make sure we ask the
provider for the value if appropriate.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/8794)