Avoid dereferencing NULL calldata pointer

This adds a check to vtkPythonCommand so that it doesn't attempt to
use the calldata pointer if the pointer is NULL.

Wrapping/PythonCore/vtkPythonCommand.cxx

@@ -137,7 +137,7 @@ void vtkPythonCommand::Execute(vtkObject *ptr, unsigned long eventtype,
                                                      callDataTypeLiteral);
 
   PyObject *arglist = NULL;
-  if (callDataTypeObj)
+  if (callData && callDataTypeObj)
   {
     if (PyInt_Check(callDataTypeObj))
     {