agent: Fix an unitialized variable in an error path.

* agent/findkey.c (agent_write_private_key): Init FP.

With
we took a shortcut but forgot to init FP which is usually set after
the new check. At least in the AppImage this always triggered a segv
and gpg or Kleopatra had to restart the agent. Not a problem in 2.4
becuase this code is a hack we don't need in 2.4 due to that new
ephemeral key storing feature.

• Fixes-commit: 3b69d8bf7146b8d10737d0cfea9c97affc60ad73
• GnuPG-bug-id: T6944