agent: Fix returning GPG_ERR_NOT_FOUND wrongly.
* agent/learncard.c (agent_handle_learn): Find SERIALNO.
Bug is: "gpg-connect-agent learn /bye" just fails wrongly.
- Fixes-commit: 8c8ce8711d9c938fcb982b0341e6b052742cb887
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>