Fix potential vtkUnicodeStringArray instability.
Fix potential vtkUnicodeStringArray instability.

There were some places in the code where a "size_t" temporary was
used to hold a value that could be negative.  I believe that this
was the cause of a failure of TestUnicodeStringArrayAPI.

Change-Id: I3bfe4556c47aa21dfb7e2288968843f4377606ac

johnkit/vtk-dev,gram526/VTK,msmolens/VTK,candy7393/VTK,hendradarwin/VTK,johnkit/vtk-dev,gram526/VTK,hendradarwin/VTK,sankhesh/VTK,msmolens/VTK,sankhesh/VTK,hendradarwin/VTK,ashray/VTK-EVM,mspark93/VTK,mspark93/VTK,SimVascular/VTK,SimVascular/VTK,msmolens/VTK,ashray/VTK-EVM,keithroe/vtkoptix,msmolens/VTK,johnkit/vtk-dev,candy7393/VTK,candy7393/VTK,SimVascular/VTK,jmerkow/VTK,ashray/VTK-EVM,SimVascular/VTK,jmerkow/VTK,msmolens/VTK,demarle/VTK,ashray/VTK-EVM,sumedhasingla/VTK,mspark93/VTK,sankhesh/VTK,mspark93/VTK,sankhesh/VTK,sankhesh/VTK,keithroe/vtkoptix,johnkit/vtk-dev,mspark93/VTK,SimVascular/VTK,jmerkow/VTK,keithroe/vtkoptix,jmerkow/VTK,msmolens/VTK,sankhesh/VTK,demarle/VTK,mspark93/VTK,ashray/VTK-EVM,demarle/VTK,sumedhasingla/VTK,msmolens/VTK,sumedhasingla/VTK,mspark93/VTK,keithroe/vtkoptix,sumedhasingla/VTK,msmolens/VTK,demarle/VTK,keithroe/vtkoptix,candy7393/VTK,jmerkow/VTK,candy7393/VTK,sumedhasingla/VTK,ashray/VTK-EVM,candy7393/VTK,jmerkow/VTK,demarle/VTK,ashray/VTK-EVM,mspark93/VTK,gram526/VTK,hendradarwin/VTK,gram526/VTK,keithroe/vtkoptix,SimVascular/VTK,candy7393/VTK,candy7393/VTK,johnkit/vtk-dev,sankhesh/VTK,sankhesh/VTK,sumedhasingla/VTK,ashray/VTK-EVM,hendradarwin/VTK,jmerkow/VTK,gram526/VTK,sumedhasingla/VTK,jmerkow/VTK,demarle/VTK,johnkit/vtk-dev,gram526/VTK,gram526/VTK,sumedhasingla/VTK,keithroe/vtkoptix,johnkit/vtk-dev,hendradarwin/VTK,demarle/VTK,hendradarwin/VTK,SimVascular/VTK,demarle/VTK,gram526/VTK,keithroe/vtkoptix,SimVascular/VTK