small fix: only perform anti-overhang when there is anti-overhang (CURA-3006, CURA-2077)
small fix: only perform anti-overhang when there is anti-overhang (CURA-3006, CURA-2077)

ROBO3D/CuraEngine,Ultimaker/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine