bug#46904: Non-unique windows produced by window-state-put

[Juri Linkov]

>> So better to fix callers not to use frame-root-window
>> that is unreliable: sometimes frame-root-window returns
>> an internal window, sometimes a live window:
>
> To emulate `set-window-configuration' via `window-state-put' you have
> to put the state into the root window.

When 'frame-root-window' returns an internal window,
then we don't put the state into the root window anyway,
we create a new window.  The patch does the same for the
case when there is only one window too, so both cases
will provide the same consistent result.