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, it is verified in ....
https://github.com/qgis/QGIS/blob/master/src/core/qgsmaptopixelgeometrysimplifier.cpp#L274
and
https://github.com/qgis/QGIS/blob/master/src/core/qgsmaptopixelgeometrysimplifier.cpp#L288