bpo-29802: Fix the cleaning up issue in PyUnicode_FSDecoder(). (#1217)

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 7871043..4e0c663 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3902,6 +3902,7 @@
     PyObject *output = NULL;
     if (arg == NULL) {
         Py_DECREF(*(PyObject**)addr);
+        *(PyObject**)addr = NULL;
         return 1;
     }