bug#47244: 28.0.50; SIGSEGV in long-runnning Emacs

[Eli Zaretskii]

> From: Michael Welsh Duggan <mwd@cert.org>
> Cc: "mwd@md5i.com" <mwd@md5i.com>,
>         "47244@debbugs.gnu.org"
>  <47244@debbugs.gnu.org>
> Date: Thu, 18 Mar 2021 15:24:42 -0400
> 
> (gdb) p XWINDOW(selected_window)->contents
> $10 = XIL(0x555558df4845)
> (gdb) xtype
> Lisp_Vectorlike
> PVEC_BUFFER
> (gdb) xbuffer
> $11 = (struct buffer *) 0x555558df4840
> "DEAD"

So the selected window's buffer was killed.  How is that possible?