agent: Allow only specific digest size for ECDSA.
* agent/pksign.c (do_encode_dsa): Fix validation of digest size.
Thanks to Steven Noonan <steven@uplinklabs.net> who offers patches
and a test case.
- GnuPG-bug-id: T2702
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>