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.
All these changes look dangerously to me: we must not quietly ignore failures during GEOS geometry creation - we should return null geometry (+cleanup partial geometries) in case of errors. Otherwise we end up returning geometries that are not equivalent to the ones asked by the developer!