Fixup bugs in vtkBoxClipDataSet.

There were some copy/paste errors and an uninitialized
traversal causing the clipped output to produce a data set
with more cell types than cell connections.

Change-Id: Ia8918b58f925c075eed10e8157406759cd789f42
Merge topic 'clip-dataset-bugs' into master

e51c5cdf Fixup bugs in vtkBoxClipDataSet.