agent: Fix agent_update_private_key.
* agent/findkey.c (agent_update_private_key): Check FNAME0.
Cherry-pick master commit of:
08e529fa7cfa8f55256337dd525fe8724c78cd92
- Fixes-commit: a216e9c028ee389c4bf0250b822d567ffe9ad85e
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>