bug#17170: 24.3.50; debug: Terminal 3 is locked, cannot read from it

[martin rudalics]

> ;; I did:
> (setq debugger-previous-window (selected-window))
>
> ;; and it went away:
> (debug)
> ;; works fine
>
> So emacs sometimes tries to use the initial frame F1. That's not good because that 
frame doesn't really exist (it's the frame created by "emacs --daemon").

When it happens again please tell me the value of
`debugger-previous-window'.  AFAICT this can only happen when that
window was shown on the "initial frame F1" before.

martin