When loading SSH-2 key, ignore passphrase argument if key is unencrypted.

When loading SSH-2 key, ignore passphrase argument if key is unencrypted.
This should get rid of a problem that three or four people reported where
PuTTY intermittently reports "Unable to load private key" (MAC failed).

(ssh.c:do_ssh2_authconn() should also initialise its passphrase so it's not
passing garbage passphrases around, of course, but I haven't yet worked out
where the best place in the auth loop to do that would be.)

[originally from svn r3439]