crypto: fix error check on gcry_md_open

Gcrypt does not return negative values on error, it returns non-zero
values. This caused QEMU not to detect failure to open an unsupported
hash, resulting in a later crash trying to use a NULL context.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>