bug#19774: 25.0.50; Emacs crashes when C-u <N> on a Gnus folder (reproducible)

[Eli Zaretskii <eliz@gnu.org>]

> From: Sebastien Vauban <sva-news@mygooglest.com>
> Cc: 19774@debbugs.gnu.org
> Date: Thu, 05 Feb 2015 11:35:34 +0100
> 
> > Any hope to get a reproducible recipe?
> 
> Wow, that was /really/ hard, but adding the following customization to
> your .emacs should allow you to reproduce it:
> 
> --8<---------------cut here---------------start------------->8---
> (add-to-list 'load-path "~/.emacs.d/elpa/key-chord-20140929.2246")
> (require 'key-chord)
> (key-chord-mode 1)                      ; NEEDED!
> (with-eval-after-load "key-chord"
>   (key-chord-define-global "x1" 'delete-other-windows)) ; NEEDED!
> 
> (add-to-list 'load-path "~/.emacs.d/elpa/guide-key-20150107.2235")
> (add-to-list 'load-path "~/.emacs.d/elpa/dash-20141220.1452")
> (add-to-list 'load-path "~/.emacs.d/elpa/popwin-20141227.2351")
> (add-to-list 'load-path "~/.emacs.d/elpa/s-20140910.334")
> (require 'guide-key)
> (guide-key-mode 1)                      ; NEEDED!
> --8<---------------cut here---------------end--------------->8---
> 
> (I don't think the versions really matter, except for Key-chord; hence,
> you could remove the add-to-list if you use `package' to load both
> `key-chord' and `guide-key'.)
> 
> After that, do:
> 
> 1. Start up Gnus
> 2. Go onto a folder
> 3. Press `C-u 1' (the "1" seems to be important too; no problem with "9")
> 

Thanks, will try that soon.

Meanwhile, does anyone has an idea how come this happened:

> #5  0x0117b144 in allocate_vectorlike (len=-1) at C:/emacs/repo/src/alloc.c:3117
                                        ^^^^^^^^
> #6  0x0117b219 in allocate_vector (len=-1) at C:/emacs/repo/src/alloc.c:3157
                                     ^^^^^^
> #7  0x01103e28 in make_uninit_vector (size=-1) at C:/emacs/repo/src/lisp.h:3789
                                        ^^^^^^^
> #8  0x0117b418 in Fvector (nargs=-1, args=0x158bcd0 <dumped_data+32528>)
                             ^^^^^^^^
>     at C:/emacs/repo/src/alloc.c:3224
> #9  0x0111d977 in Fthis_single_command_keys () at C:/emacs/repo/src/keyboard.c:10054

It looks like this_command_key_count is -1.  How can that happen?
Should we protect ourselves against this?