ssh-keysign: fix double free in error path
From OpenSSH-portable commit 141535b904b6, OpenBSD commit 3d21aa127b1f.
MFC after: 3 days
(cherry picked from commit 963c7b084b8bf2397deced4f568b976a42611910)
(cherry picked from commit 6dca51b9f32dd3f6a0dd58a0f1525db3d27f485d)