Fix gcc warning about possible use of uninitialized variable

As best as I can tell, in regular use, it's basically a false positive,
but I think cases could be contrived where the compiler is right.

/Common/DataModel/vtkPolyhedron.cxx: In member function 'int vtkPolyhedron::vtkInternal::OrderDisconnectedContourPoints(vtkIdSetType&, vtkPoints*, vtkIdVectorType&, vtkIdVectorType&)':
/Common/DataModel/vtkPolyhedron.cxx:1195:19: warning: 'o' may be used uninitialized in this function [-Wmaybe-uninitialized]
       e[0] = x[0] - o[0];
                   ^

Defensively added initialization to zero in the error case.

Also fixed some comments, added some const and static.

Change-Id: Id2b6e8d69a07c7f248b7656b791d57ff9a92ccf4
Merge topic 'vtkPolyhedron-uninit' into master

8446c7b0 Fix gcc warning about possible use of uninitialized variable