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 @wonder-sk, IMHO I think that the code added in "QgsSymbolLayerV2Utils::polygonPointOnSurface" above is needed. If you test the data attached to the issue, do you not see a red point in the left-top of the map window ? I think that "QgsSymbolLayerV2Utils::polygonCentroid" returns a empty point in some situations.