bpo-35356: Fix a possible reference leak in nis.maps(). (GH-10808)

(cherry picked from commit a2e3585e)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Modules/nismodule.c

@@ -405,6 +405,7 @@ nis_maps (PyObject *self, PyObject *args, PyObject *kwdict)
         PyObject *str = PyString_FromString(maps->map);
         if (!str || PyList_Append(list, str) < 0)
         {
+            Py_XDECREF(str);
             Py_DECREF(list);
             list = NULL;
             break;