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.
Unfortunately in this case neither of these conditions help: in first case because hasUngenerizableSegments=true, in the second case because first and last vertex are the same.