don't crash if GeoSceneDocument::head() returns zero
don't crash if GeoSceneDocument::head() returns zero

AndreiDuma/marble,rku/marble,probonopd/marble,quannt24/marble,tucnak/marble,tzapzoor/marble,adraghici/marble,David-Gil/marble-dev,oberluz/marble,adraghici/marble,tzapzoor/marble,David-Gil/marble-dev,AndreiDuma/marble,David-Gil/marble-dev,David-Gil/marble-dev,probonopd/marble,adraghici/marble,Earthwings/marble,quannt24/marble,adraghici/marble,rku/marble,adraghici/marble,tucnak/marble,utkuaydin/marble,rku/marble,utkuaydin/marble,David-Gil/marble-dev,tucnak/marble,tucnak/marble,utkuaydin/marble,AndreiDuma/marble,tzapzoor/marble,tzapzoor/marble,Earthwings/marble,rku/marble,tzapzoor/marble,rku/marble,Earthwings/marble,quannt24/marble,tucnak/marble,tucnak/marble,Earthwings/marble,rku/marble,tzapzoor/marble,AndreiDuma/marble,oberluz/marble,utkuaydin/marble,oberluz/marble,Earthwings/marble,David-Gil/marble-dev,Earthwings/marble,quannt24/marble,AndreiDuma/marble,probonopd/marble,utkuaydin/marble,oberluz/marble,tucnak/marble,AndreiDuma/marble,quannt24/marble,adraghici/marble,probonopd/marble,oberluz/marble,probonopd/marble,oberluz/marble,probonopd/marble,probonopd/marble,quannt24/marble,tzapzoor/marble,quannt24/marble,tzapzoor/marble,utkuaydin/marble