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 <wk@gnupg.org>