Guard against infill density being 0
Guard against infill density being 0

Otherwise the logarithm is undefined

Contributes to issue CURA-3137.

hmflash/Cura,fieldOfView/Cura,hmflash/Cura,ynotstartups/Wanhao,fieldOfView/Cura,ynotstartups/Wanhao,Curahelper/Cura,Curahelper/Cura