agent: Flush before calling ftruncate.

* agent/findkey.c (write_extended_private_key): Make sure
it is flushed out.

--

Cherry-picked from master commit of:
	99d2931887

GnuPG-bug-id: 6035
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

agent/findkey.c

@@ -111,6 +111,8 @@ write_extended_private_key (char *fname, estream_t fp, int update, int newkey,
     goto leave;
 
   err = nvc_write (pk, fp);
+  if (!err)
+    err = es_fflush (fp);
   if (err)
     {
       log_error ("error writing '%s': %s\n", fname, gpg_strerror (err));