bug#48408: BUGFIX: window-default-font-height: check for nil string

[martin rudalics]

>  > In package windows.el, function `window-default-font-height' calls
>  > function `face-font', which can return a nil value, but performs a
>  > `string-equal' operation on the output without checking for that
>  > possibiliy.
>
> Please report this to the author(s) of that package.

Sorry.  This function is missing from my current window.el here ...

>  > I came across this in an unusual case, of having an 'emacs -nw' server
>  > an 'emacsclient -nw', and opening a GUI emacsclient to connect to it.
>  >  From that point forward until the fix, every *second* attempt to use the
>  > minibuffer on the 'emacsclient -nw' would fail.
>  >
>  > The fix I'm using is:
>  >
>  >      (if (and (display-multi-font-p (frame-parameter frame 'display))
>  >               (stringp default-font)  ; <<<---!!!
>
> What is `default-font'?

... and so I didn't catch this one either.  I'll try to find out what
I'm doing differently now.

martin