agent: Skip unknown unknown ssh curves seen on cards.

* agent/command-ssh.c (ssh_handler_request_identities): Skip unknown
curves.

For example when using my standard ed25519 token and testing cards
with only Brainpool support, the ssh-agent failed due to the unknown
curves seen on the card. This patches fixes this by ignoring keys
with unknown curves.

• Signed-off-by: Werner Koch <wk@gnupg.org>