Limit the max_value for extruders to the number of extruders
Limit the max_value for extruders to the number of extruders

CuraEngine would crash if instructed to use an extruder that it knows nothing about. Extruders start counting at 0, so max_value is set to machine_extruder_count - 1.

Fixes the crash outlined in CURA-965

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