bug#73307: Fix ctype(3) usage

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 16 Sep 2024 23:25:16 +0200
> From: Thomas Klausner <wiz@gatalith.at>
> 
> When compiling emacs on NetBSD-10.99.12/amd64, I get the following
> warning
> 
> In file included from /usr/include/ctype.h:100,
>                  from fns.c:29:
> fns.c: In function 'Fyes_or_no_p':
> fns.c:3582:33: warning: array subscript has type 'char' [-Wchar-subscripts]
>  3582 |     if ((len > 0) && !isspace (s[len - 1]))
>       |                                 ^
> 
> The NetBSD man page for ctype(3): https://man.netbsd.org/ctype.3
> is quite explicit about the problems with this - ctype(3) functions only
> accept -1 and "unsigned char" and you can get very weird problems if
> this is disregarded.
> 
> The attached patch adds the missing cast.
> [...]
>
> --- src/fns.c.orig	2024-09-16 21:11:40.908684144 +0000
> +++ src/fns.c
> @@ -3579,7 +3579,7 @@ by a mouse, or by some window-system ges
>    {
>      char *s = SSDATA (prompt);
>      ptrdiff_t len = strlen (s);
> -    if ((len > 0) && !isspace (s[len - 1]))
> +    if ((len > 0) && !isspace ((unsigned char)s[len - 1]))
>        prompt = CALLN (Fconcat, prompt, build_string (" "));
>    }
>    prompt = CALLN (Fconcat, prompt, Vyes_or_no_prompt);

Thanks.  However, I'm not sure this is the right fix, the function is
defined with argument type of 'int'.  Paul, any comments?