crypto: afalg: fix a NULL pointer dereference

Test-crypto-hash calls qcrypto_hash_bytesv/digest/base64 with
errp=NULL, this will cause a NULL pointer dereference if afalg_driver
doesn't support requested algos:

    ret = qcrypto_hash_afalg_driver.hash_bytesv(alg, iov, niov,
                                                result, resultlen,
                                                errp);
    if (ret == 0) {
        return ret;
    }

    error_free(*errp);  // <--- here

Because the error message is thrown away immediately, we should
just pass NULL to hash_bytesv(). There is also the same problem in
afalg-backend cipher & hmac, let's fix them together.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Longpeng <longpeng2@huawei.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
