fix strncat and wcsncat (double null termination)

also modify wcsncpy to use the same loop logic

src/string/wcsncpy.c

@@ -3,7 +3,7 @@
 wchar_t *wcsncpy(wchar_t *d, const wchar_t *s, size_t n)
 {
 	wchar_t *a = d;
-	for (; n && (*d = *s); n--, d++, s++);
+	while (n && *s) n--, *d++ = *s++;
 	wmemset(d, 0, n);
 	return a;
 }