Fix invalid read in vtkNewickTreeReader

This defect crept in during my recent work to improve the
vtkNewickTreeReader's performance while reading large trees.

Change-Id: I1a575adeca304a3a0b94aa3a7448bf6327ca6aea
Merge topic 'fix_Newick_valgrind_defect' into master

8c745664 Fix invalid read in vtkNewickTreeReader