Error mapping point fields in Contour

The Contour filter sometimes has out-of-bounds array access when mapping point fields. The error can be encountered when using the BenchmarkFilters with a small wavelet generated. The following command line tickles the bug.

BenchmarkFilters --device=Any --wavelet-dim=10 --benchmark_min_time=0

You might need to use a debug compile to get alerted to the issue. (Debug builds use a VTKM_ASSERT to check the bounds of array access.)