fix: inner (2nd) wall was refered to even if it wasn't generated (CURA-1294)
fix: inner (2nd) wall was refered to even if it wasn't generated (CURA-1294)

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