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.
It returns (nan, nan) because the polygon area is empty so it's not possible to calculate the centroid. Again, the problem is due to the invalid input. Let's fix the simplification - otherwise we will have similar problems in various other pieces of code in QGIS.