This attached program failed when last character of string is alphabet or number.
I'm running in ko_KR.UTF-8 locale.

for example, 

$ locale
LANG=ko_KR.UTF-8
LC_CTYPE="ko_KR.UTF-8"
LC_NUMERIC="ko_KR.UTF-8"
LC_TIME="ko_KR.UTF-8"
LC_COLLATE="ko_KR.UTF-8"
LC_MONETARY="ko_KR.UTF-8"
LC_MESSAGES="ko_KR.UTF-8"
LC_PAPER="ko_KR.UTF-8"
LC_NAME="ko_KR.UTF-8"
LC_ADDRESS="ko_KR.UTF-8"
LC_TELEPHONE="ko_KR.UTF-8"
LC_MEASUREMENT="ko_KR.UTF-8"
LC_IDENTIFICATION="ko_KR.UTF-8"
LC_ALL=
$./swprintfbug °¡³ª´Ù
°¡³ª´Ù
$ ./swprintfbug 123
Error occured in swprintf
$ ./swprintfbug abc
Error occured in swprintf
$ ./swprintfbug 123°¡³ª´Ù
123°¡³ª´Ù