bug#32850: 27.0.50; window-swap-states doesn't swap window prev/next-buffers

[martin rudalics]

> I found a problem, it occurs very rarely:
>
> Debugger entered--Lisp error: (wrong-type-argument stringp nil)
>    get-buffer(nil)
>    #f(compiled-function (entry) #<bytecode 0x22c739>)((nil nil nil))
>    mapcar(#f(compiled-function (entry) #<bytecode 0x22c739>) (("*Group*" 1 139) (nil 
nil nil) ("*sent reply to martin rudalics*" 1 1090)))
>    window--state-put-2(safe 737)
>    window-state-put(...)
>
> Do you have an idea how did (nil nil nil) find its way into the list of
> prev-buffers?  (window-state-get was used with the non-nil WRITABLE arg)

The most simple way to find out is to define such a list in window.c
and have 'set-window-prev-buffers' holler if the prev_buffers argument
is equal to that list.

martin