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
Merge topic 'negative-unsigned' into master

e0292d02 Fix potential vtkUnicodeStringArray instability.