ssh-keygen: fix double free in error path

From OpenSSH-portable commit 5062ad48814b, OpenBSD commit 39f35e16ba87.

MFC after: 3 days

(cherry picked from commit 666605ad2df3f5c1fb973dbd719ffbe3f3935c1d)
(cherry picked from commit 02c646bb7288986912ad7bbe410636d0e257eda9)