vtkStructuredCellArray: Fix an int overflow
Fix an int overflow in the connectivity backend that could notably occur when using vtkStructuredCellArray::GetCellAtId; and did occur in vtkImageData::GetCellPoints with (very) large images.
Fix an int overflow in the connectivity backend that could notably occur when using vtkStructuredCellArray::GetCellAtId; and did occur in vtkImageData::GetCellPoints with (very) large images.