Fix bug #11489: Crash with force point inside polygon in centroid fill style

This PR fixes the issue #11489 ( http://hub.qgis.org/issues/11489 )

It avoids calculate a point inside invalid polygons (maybe after simplification) using GEOS.
Hi @ahuarte47 I believe the 3-point polygon rings should be handled in the simplification code as the linear rings with just three points cannot be valid... (first and last vertex has to be the same, so you need at least two more vertices to form a proper ring)