Fix bug `id` and `pw` are not cleared on fail

If user failed to login once, user can't login even if the inputs are
correct. Passing 'off' as id won't work, too.

This bug is introduced in 36101d8821e30948482eec1aac90082cbe474940.
Merge pull request #48 from yous/patch-login-bug

Fix bug `id` and `pw` are not cleared on login fail