Avoid to incref NULL cypclass objects

Cython/Utility/ModuleSetupCode.c

@@ -1557,7 +1557,7 @@ static void __Pyx_FastGilFuncInit(void) {
     /* Cast argument to CyObject* type. */
     #define _CyObject_CAST(op) op
 
-    #define Cy_INCREF(op) _Cy_INCREF(_CyObject_CAST(op))
+    #define Cy_INCREF(op) do {if (op != NULL) {_Cy_INCREF(_CyObject_CAST(op));}} while(0)
     #define Cy_DECREF(op) do {if (_Cy_DECREF(_CyObject_CAST(op))) {op = NULL;}} while(0)
     #define Cy_XDECREF(op) do {if (op != NULL) {Cy_DECREF(op);}} while(0)
     #define Cy_GOTREF(op)