Fixed bug in vtkMaskPoints that caused corrupt output.

When MaximumNumberOfPoints was set to 0, vtkMastPoints produced
a polydata with 1 cell but no points.

Change-Id: I23360009b1f66dc92349e3726b3eae069d7533ad
Merge topic 'mask-points-fix' into master

86bd7ac8 Fixed bug in vtkMaskPoints that caused corrupt output.