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]

sshpubk.c

@@ -784,7 +784,7 @@ struct ssh2_userkey *ssh2_load_userkey(const Filename *filename,
 
 	    SHA_Init(&s);
 	    SHA_Bytes(&s, header, sizeof(header)-1);
-	    if (passphrase)
+	    if (cipher && passphrase)
 		SHA_Bytes(&s, passphrase, passlen);
 	    SHA_Final(&s, mackey);