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

(cherry picked from commit a2e3585e79c93b2372dbad46a744e28fcc6dad6d)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>
diff --git a/Modules/nismodule.c b/Modules/nismodule.c
index a9028bb..11df679 100644
--- a/Modules/nismodule.c
+++ b/Modules/nismodule.c
@@ -412,6 +412,7 @@
         PyObject *str = PyUnicode_FromString(maps->map);
         if (!str || PyList_Append(list, str) < 0)
         {
+            Py_XDECREF(str);
             Py_DECREF(list);
             list = NULL;
             break;