Add safety check against bad card certificate.
Add safety check against bad card certificate.

* src/agent.c (scute_agent_get_cert): Reject card certificate if
it does not start with an ASN.1 sequence tag.

Signed-off-by: Damien Goutte-Gattat <3a260918a4ae0abc8ac3e544c6df1814a2596a5f@incenp.org>

gpg/scute,gpg/scute,gpg/scute