Resolves a runtime with alt icons.

The runtime occurred when get_icon_state() attempted to handle_alt_icon
on a head, even if if was a stump. Adds sanity to check if it's a stump
or if the organ's destroyed (as how it's handled in update_icons for
mobs) and resolves the runtime and #5863.

code/modules/surgery/organs/organ_icon.dm

@@ -159,7 +159,7 @@ var/global/list/limb_icon_cache = list()
 				gender = "f"
 			else
 				gender = "m"
-		if(limb_name == "head")
+		if(limb_name == "head" && !is_stump() && !(status & ORGAN_DESTROYED))
 			var/obj/item/organ/external/head/head_organ = src
 			head_organ.handle_alt_icon()