BUG: Fix issue 2992: Crash when MRML nodes registered to a vtkMRMLIGTLConnectorNode is deleted.

From: Junichi Tokuda <tokuda@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21983 3bd1e089-480b-0410-8dfb-8563597acbee