bugfix: DIV_BY_ZERO in MatchingFunctionSpacePartitionerLonLatPolygon …

…for small grids with OMP (#244)

src/atlas/grid/detail/partitioner/MatchingFunctionSpacePartitionerLonLatPolygon.cc

@@ -56,6 +56,9 @@ void MatchingFunctionSpacePartitionerLonLatPolygon::partition(const Grid& grid,
             ATLAS_TRACE("point-in-polygon check for entire grid (" + std::to_string(grid.size()) + " points)");
             size_t num_threads = atlas_omp_get_max_threads();
             size_t chunk_size  = grid.size() / (1000 * num_threads);
+            if (chunk_size == 0) {
+                chunk_size = grid.size();
+            }
             size_t chunks      = num_threads == 1 ? 1 : std::max(size_t(1), size_t(grid.size()) / chunk_size);
             atlas_omp_pragma(omp parallel for schedule(dynamic,1))
             for( size_t chunk=0; chunk < chunks; ++chunk) {