Fix PExodusIIWriter called without GlobalMPController.

If you enable the Module_vtkIOParallelExodus option, the object factory for a vtkExodusIIWriter will return a vtkPExodusIIWriter. However this parallel version suppose a parallel use of the writer and a global MultiProcessController to be set which is not the case if you use the writer in serial.

This patch ensures that the writer will not crash and will have the same behaviour than the (hidden) serial writer.