Guard helper function against missing usage when direct type slot access is disabled.

Cython/Utility/Coroutine.c

@@ -5,12 +5,14 @@ static CYTHON_INLINE PyObject* __Pyx_Generator_Yield_From(__pyx_CoroutineObject
 //////////////////// GeneratorYieldFrom ////////////////////
 //@requires: Generator
 
+#if CYTHON_USE_TYPE_SLOTS
 static void __PyxPyIter_CheckErrorAndDecref(PyObject *source) {
     PyErr_Format(PyExc_TypeError,
                  "iter() returned non-iterator of type '%.100s'",
                  Py_TYPE(source)->tp_name);
     Py_DECREF(source);
 }
+#endif
 
 static CYTHON_INLINE PyObject* __Pyx_Generator_Yield_From(__pyx_CoroutineObject *gen, PyObject *source) {
     PyObject *source_gen, *retval;