sm: Support verification of nistp521 signatures.
* sm/certcheck.c (do_encode_md): Take care of nistp521.
That curve is a bit odd in that it does not match a common hash digest
length. We fix that here for just this case instead of writing more
general code to support all allowed cases (i.e. hash shorter than Q).
- Signed-off-by: Werner Koch <firstname.lastname@example.org>