Fixed reference counting bug in vtkTransform

There was a reference counting error in vtkTransformConcatenation when
a transform was DeepCopy'ed and shortened and then DeepCopy'ed and lengthened
using some of the same concatenated transforms.

When the list of concatenated transforms was shortened, the values
were not cleared out, letting pointers in the list that did not have a reference
according to the reference counting.  Later, when the list was set to a larger
list again and happenned to have the same transform at postion x, since there
was already a reference to the transform in the list, it did not increment
the reference count even though it should have.  This commit fixes this
by setting pointers beyond the end of the list to NULL when DeepCopy shrinks the list.

This commit also adds a regression test for that will segfault unless the
bug is fixed due to a reference count that is accidentally decremented in
a loop due to the bug. Running this test without this patch will segfault.

Change-Id: I27d7c2097f85a2f8c1d6e8a0e8213d041cc83125
Merge topic 'transform_reference_counting_bug' into master

b47332bb Fixed reference counting bug in vtkTransform