scd: Add condition for VERIFY with 0x82.

* scd/app-openpgp.c (verify_chv2): Check availability of keys in
question.

With buggy Gnuk (<= 1.2.15), when no encr/auth keys are available,
it fails decrementing the signature error counter. This change
can avoid the issue.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>