bug#56443: 28.1; Mac segmentation fault with ffap

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Fri, 8 Jul 2022 11:35:27 +0200
> Cc: Daniel Martín <mardani29@yahoo.es>, 56443@debbugs.gnu.org,
>         sje30@cam.ac.uk
> 
> The call to file_name_directory returns nil (the argument being 
> "mailto:sje30@cam.ac.uk";). What about:
> 
> @@ -2603,7 +2603,7 @@ DEFUN ("file-name-case-insensitive-p", 
> Ffile_name_case_insensitive_p,
>        Lisp_Object parent = file_name_directory (filename);
>        /* Avoid infinite loop if the root has trouble
>          (impossible?).  */
> -      if (!NILP (Fstring_equal (parent, filename)))
> +      if (!STRINGP (parent) || !NILP (Fstring_equal (parent, filename)))
>         return Qnil;
>        filename = parent;
>      }

Fine with me, but please update the comment to this effect.

But why does the original call to file_name_case_insensitive_err fail
in this case with err > 0 on macOS, and what is the value of err?