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

Author: serhiy-storchaka

Objects/unicodeobject.c

@@ -3902,6 +3902,7 @@ PyUnicode_FSDecoder(PyObject* arg, void* addr)
     PyObject *output = NULL;
     if (arg == NULL) {
         Py_DECREF(*(PyObject**)addr);
+        *(PyObject**)addr = NULL;
         return 1;
     }