bug#39482: 26.3; set-fontset-font with emacs charset does not work correctly

[Eli Zaretskii]

> From: ynyaaa@gmail.com
> Cc: 39482@debbugs.gnu.org
> Date: Sat, 08 Feb 2020 11:27:53 +0900
> 
> #'setfontset-font does not work well if ADD argument is non-nil.
> In the example below, dummy3 should be added for both 199 and 200,
> but it is not added for 200.
> 
> (progn (set-fontset-font t 199 "dummy1")
>        (set-fontset-font t 200 "dummy2")
>        (set-fontset-font t '(199 . 200) "dummy3" nil 'append)
>        (mapcar (lambda (c) (cons c (aref (fontset-info t) c)))
>                '(199 200)))
> => ((199 ("-*-dummy1-*-*-*-*-*-*-*-*-*-*-*-*")
>          ("-*-dummy3-*-*-*-*-*-*-*-*-*-*-*-*"))
>     (200 ("-*-dummy2-*-*-*-*-*-*-*-*-*-*-*-*")))

There was an off-by-one error in this function, which is now fixed.

Thanks.