If the reader is created without MPI initialized then no Controller is
available.  This can happen when vtkNrrdReader::New() returns a
vtkPNrrdReader through the object factory in a non-MPI process.  When no
controller is available fail gracefully rather than crashing.

Change-Id: Icc9a7c0b6f57b921db6e92b3bcc7d68e649261b0