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

[Juri Linkov]

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 gets out of this loop only on an error such as when point reaches
the beginning of the buffer that signals an error.

PS: this is the simplest reproducible case, but I found this bug
by trying to use 'C-x RET c C-u C-u M-x rgrep' that with two C-u prefixes
allows editing the grep command.