fix Newick reader for named root vertex

Before this commit, the vtkNewickTreeReader would trash if you attempted
to load a file where the root vertex of the tree was named.

Change-Id: I1d1f282fc586a4a0b5569bb5d2e065dfe6a8aa7e
Merge topic 'newick_named_root' into master

a788e7ff fix Newick reader for named root vertex