bug#13553: 24.3.50; incorrect usage of IS_DIRECTORY_SEP

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: shigeru.fukaya@gmail.com,  13553@debbugs.gnu.org
> Date: Sat, 26 Jan 2013 17:24:37 -0500
> 
> CharNextExA and CharPrevExA, in my mind, do perform the moral equivalent
> of decoding the argument (just in an incremental way).

No, they are the equivalents of NEXT_CHAR_BOUNDARY and
PREV_CHAR_BOUNDARY, except that they can do this for many different
encodings, not just for UTF-8.