iconv/wctomb: fix memory corruption related to CURRENT_UTF8 implementation (#511)

Author: Mrmaxmeier

libc-top-half/musl/src/internal/locale_impl.h

@@ -64,7 +64,7 @@ hidden char *__gettextdomain(void);
         &libc.current_locale; \
     }))
 
-#define CURRENT_UTF8 (!!libc.global_locale.cat[LC_CTYPE])
+#define CURRENT_UTF8 (!!CURRENT_LOCALE->cat[LC_CTYPE])
 #endif
 
 #undef MB_CUR_MAX