Windows: fix leak in posix_listdir.

Modules/posixmodule.c

@@ -2396,6 +2396,7 @@ posix_listdir(PyObject *self, PyObject *args)
     }
     strcpy(namebuf, PyBytes_AsString(opath));
     len = PyObject_Size(opath);
+    Py_DECREF(opath);
     if (len > 0) {
         char ch = namebuf[len-1];
         if (ch != SEP && ch != ALTSEP && ch != ':')