Fix assert in vtkRAdapter

907df31 introduced a bug that would allow
nodeWeights->GetValue(...) to be called if parent is
-1, move this code into the protected if block.

Change-Id: I9838f59dbb8b571f75ac9eb38f33114f4de45eaf
Merge topic 'fix-radapter-parent-index' into master

43d4a38b Fix assert in vtkRAdapter