bug#13553: 24.3.50; incorrect usage of IS_DIRECTORY_SEP

[Shigeru Fukaya]

Hi,

`file-attributes' returns nil as stat info for a file, its name ending
with a character whose second byte is 0x5c.

It is because IS_DIRECTORY_SEP is wrongly used.
IS_DIRECTORY_SEP only works when its argument is surely on a start
byte of dbcs characters.
I find incorrect usage, at least in some functions of `w32.c'.
(readdir, stat_worker, read_unc_volume)


I show, as a silly sample, of easy fix.

#define IS_AT_DIRECTORY_SEP(p,i) \
  (p[i] == '/' || (p[i] == '\\' && _mbsbtype(p,i) == _MBC_SINGLE))

and,

  IS_DIRECTORY_SEP(p[i]) 

to

  IS_AT_DIRECTORY_SEP(p, i) 


Regards,
Shigeru