fix leak in extract_polygonal_prism_data

The inlier-check worked before Alexandru broke it in 2012.
(see 614972920ffa1b62b7341aedeff3902419ac2afa)
This last test in the raytracing code is actually already
the _first_ one that is tested for.
Merge pull request #1044 from v4hn/fix-extract-polygonal-prism

fix leak in extract_polygonal_prism_data