decoder.c: Fix a scan-build warning.

  decoder.c:343:9: warning: Access to field 'ob_type' results in a dereference of a null pointer (loaded from variable 'py_default')
                                  if (Py_TYPE(py_default) != Py_TYPE(py_item)) {
                                      ^~~~~~~~~~~~~~~~~~~
  /usr/include/python3.6m/object.h:118:33: note: expanded from macro 'Py_TYPE'
  #define Py_TYPE(ob)             (((PyObject*)(ob))->ob_type)
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~