Fix refleak in a rare error case.

Cython/Utility/CythonFunction.c

@@ -494,6 +494,7 @@ static PyObject *__Pyx_CyFunction_Init(__pyx_CyFunctionObject *op, PyMethodDef *
         break;
     default:
         PyErr_SetString(PyExc_SystemError, "Bad call flags for CyFunction");
+        Py_DECREF(op);
         return NULL;
     }
 #endif