Use checked meshgroup in constructing SliceDataStorage
Use checked meshgroup in constructing SliceDataStorage

The rest of this constructor was still using the input meshgroup, which might be nullptr. Make it use the field, which was checked for being nullptr and initialised if it was nullptr.
ROBO3D/CuraEngine,ROBO3D/CuraEngine,totalretribution/CuraEngine,Ultimaker/CuraEngine,totalretribution/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,pratikshashroff/pcura,alephobjects/CuraEngine,pratikshashroff/pcura,pratikshashroff/pcura,totalretribution/CuraEngine