agent: Another fix for a regression with unknown curves and ssh

* agent/command-ssh.c (ssh_send_available_keys): Clear the error when
skipping.

Of course we need to clear the error so that in case the last key in
the array was skipped, that error is not returned by the function.

Updates-commit: 6bf5696c8578782a25227c0806ce5f8abcafb603
(cherry picked from commit 55db12472f1cc7034a82932f271c94e4e61bc3dd)