bug#25875: 26.0.50; Hang logging out of MS-Windows

[Eli Zaretskii]

> From: Richard Copley <rcopley@gmail.com>
> Date: Sat, 25 Feb 2017 19:41:25 +0000
> 
> Perhaps not a deadlock: I just ran "taskkill /im emacs.exe" and the
> process woke up
> and signaled a Lisp error:
> 
> Debugger entered--Lisp error: (error "Attempt to delete a surrogate
> minibuffer frame")
>   delete-frame(#<frame *Customize Option: Package Unsigned Archives*
> 0000000400a51890> t)

Is this indeed a surrogate minibuffer frame?  If so, how come it's
being deleted?