bug#38642: 'C-x RET c' gets stuck in universal-argument loop

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Date: Tue, 17 Dec 2019 00:07:25 +0100
> Cc: 38642@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
> 
> > emacs -Q
> > C-x RET c       ;; universal-coding-system-argument
> > RET             ;; accept default e.g. utf-8-unix
> > C-u C-b         ;; moves backwards 4 chars
> > C-b C-b C-b ...
> >
> > Every C-b still moves backwards 4 chars because it gets stuck with an old
> > 'prefix-arg' in the 'while' loop in 'universal-coding-system-argument'.
> 
> It makes assumptions about universal-argument-map that are no longer
> true since commit 02ef6c1a8a.

Stefan, any ideas for how to fix this unintended consequence?

TIA