Fix switchtool undeploy runtime
Created by: mph55
[05:29:15] Runtime in switchtool.dm,119: Cannot modify null.cant_drop. proc name: undeploy (/obj/item/weapon/switchtool/proc/undeploy) usr: Igor Slovenich (h0l3m4k3r) (/mob/living/carbon/human) usr.loc: The floor (295, 200, 1) (/turf/simulated/floor) src: the surgeon's switchtool (/obj/item/weapon/switchtool/surgery) src.loc: Igor Slovenich (/mob/living/carbon/human) call stack: the surgeon's switchtool (/obj/item/weapon/switchtool/surgery): undeploy() the surgeon's switchtool (/obj/item/weapon/switchtool/surgery): attack self(Igor Slovenich (/mob/living/carbon/human)) Igor Slovenich (/mob/living/carbon/human): Activate Held Object()