clip coverages: fix crash when buffered mask polygons are empty
clip coverages: fix crash when buffered mask polygons are empty

vrsource/mapproxy,mapproxy/mapproxy,drnextgis/mapproxy,camptocamp/mapproxy,camptocamp/mapproxy,olt/mapproxy,mapproxy/mapproxy,olt/mapproxy,vrsource/mapproxy,drnextgis/mapproxy