bug#28179: Fix use of string-to-multibyte in ispell.el

[Eli Zaretskii]

> From: Reuben Thomas <rrt@sc3d.org>
> Date: Tue, 22 Aug 2017 01:51:53 +0100
> 
>  ;; Return a string decoded from Nth element of the current dictionary.
>  (defun ispell-get-decoded-string (n)
>    "Get the decoded string in slot N of the descriptor of the current dict."
>    (let* ((slot (or
>               (assoc ispell-current-dictionary ispell-local-dictionary-alist)
>               (assoc ispell-current-dictionary ispell-dictionary-alist)
> -             (error "No data for dictionary \"%s\", neither in 
> `ispell-local-dictionary-alist' nor in `ispell-dictionary-alist'"
> -                    ispell-current-dictionary)))
> -      (str (nth n slot)))
> -    (when (and (> (length str) 0)
> -            (not (multibyte-string-p str)))
> -      (setq str (ispell-decode-string str))
> -      (or (multibyte-string-p str)
> -       (setq str (string-to-multibyte str))))

Are you sure we don't need to ensure ispell-get-decoded-string always
returns a multibyte string?  What if decode-coding-string returns a
pure ASCII string, which is therefore unibyte?

IOW, it looks like you didn't replace the call to string-to-multibyte
with something equivalent, you simply removed that call.  If you
analyzed the code and concluded that this call is redundant, please
tell the details of your analysis.

Thanks.