fix: mergeInfillLines aborts if applied to a print feature other than infill or skin (CURA-581)
fix: mergeInfillLines aborts if applied to a print feature other than infill or skin (CURA-581)

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