BUGFIX: Add guards for NULL ivars in PrintSelf

The PrintSelf method of vtkStructuredImplicitConnectivity
was causing failures in the otherPrint test b/c it was
dereferencing NULL ivars. Fixed that by adding guards
to chekc if the ivars are NULL.

Change-Id: I50416653e14d5af50a6408c09129f1deebf63ebf
Merge topic 'fix-otherPrint' into master

33920cf2 BUGFIX: Add guards for NULL ivars in PrintSelf