Fix the single wall as no sparse infill bug.
Fix the single wall as no sparse infill bug.

mspark93/CuraEngine,pratikshashroff/pcura,Ultimaker/CuraEngine,Ultimaker/CuraEngine,be3d/CuraEngine,derekhe/CuraEngine,uus169/CuraEngine,alex1818/CuraEngine,uus169/CuraEngine,be3d/CuraEngine,be3d/CuraEngine,robotustra/curax,derekhe/CuraEngine,Intrinsically-Sublime/CuraEngine,foosel/CuraEngine,jacobdai/CuraEngine-1,alephobjects/CuraEngine,fxtentacle/CuraEngine,fxtentacle/CuraEngine,patrick3coffee/CuraTinyG,daid/CuraCutEngine,mspark93/CuraEngine,markwal/CuraEngine,electrocbd/CuraEngine,robotustra/curax,totalretribution/CuraEngine,Jwis921/PersonalCuraEngine,foosel/CuraEngine,alephobjects/CuraEngine,fxtentacle/CuraEngine,Skeen/CuraJS-Engine,daid/CuraCutEngine,Intrinsically-Sublime/CuraEngine,jacobdai/CuraEngine-1,patrick3coffee/CuraTinyG,robotustra/curax,totalretribution/CuraEngine,uus169/CuraEngine,Skeen/CuraJS-Engine,Intrinsically-Sublime/CuraEngine,markwal/CuraEngine,ROBO3D/CuraEngine,patrick3coffee/CuraTinyG,phonyphonecall/CuraEngine,Skeen/CuraJS-Engine,alex1818/CuraEngine,foosel/CuraEngine,pratikshashroff/pcura,pratikshashroff/pcura,electrocbd/CuraEngine,phonyphonecall/CuraEngine,markwal/CuraEngine,totalretribution/CuraEngine,jacobdai/CuraEngine-1,alephobjects/CuraEngine,mspark93/CuraEngine,electrocbd/CuraEngine,Jwis921/PersonalCuraEngine,phonyphonecall/CuraEngine,ROBO3D/CuraEngine,derekhe/CuraEngine,alex1818/CuraEngine,ROBO3D/CuraEngine,Jwis921/PersonalCuraEngine