Fixes runtime when thermiting a wall

Fixes not being able to put down a chair on a tile with a border window.
Fixes Borg module cycling not working when the first module is empty.

code/game/objects/structures/beds_chairs/chair.dm

@@ -253,7 +253,7 @@
 		if(istype(A,/obj/structure/chair))
 			user << "<span class='danger'>There is already a chair here.</span>"
 			return
-		if(A.density)
+		if(A.density && !(A.flags & ON_BORDER))
 			user << "<span class='danger'>There is already something here.</span>"
 			return
 

code/game/turfs/simulated/walls.dm

@@ -226,11 +226,12 @@
 	playsound(src, 'sound/items/Welder.ogg', 100, 1)
 
 	if(thermite >= 50)
+		var/burning_time = max(100,300 - thermite)
 		var/turf/open/floor/F = ChangeTurf(/turf/open/floor/plating)
 		F.burn_tile()
 		F.icon_state = "wall_thermite"
 		F.add_hiddenprint(user)
-		spawn(max(100,300-thermite))
+		spawn(burning_time)
 			if(O)
 				qdel(O)
 	else

code/modules/mob/living/silicon/robot/inventory.dm

@@ -198,25 +198,25 @@
 //cycle_modules() - Cycles through the list of selected modules.
 /mob/living/silicon/robot/proc/cycle_modules()
 	var/slot_start = get_selected_module()
-	if(slot_start) deselect_module(slot_start) //Only deselect if we have a selected slot.
+	if(slot_start)
+		deselect_module(slot_start) //Only deselect if we have a selected slot.
 
 	var/slot_num
 	if(slot_start == 0)
 		slot_num = 1
-		slot_start = 2
+		slot_start = 4
 	else
 		slot_num = slot_start + 1
 
-	while(slot_start != slot_num) //If we wrap around without finding any free slots, just give up.
+	while(slot_num != slot_start) //If we wrap around without finding any free slots, just give up.
 		if(module_active(slot_num))
 			select_module(slot_num)
 			return
 		slot_num++
-		if(slot_num > 3) slot_num = 1 //Wrap around.
+		if(slot_num > 4) // not >3 otherwise cycling with just one item on module 3 wouldn't work
+			slot_num = 1 //Wrap around.
 
-	return
 
 
 /mob/living/silicon/robot/swap_hand()
 	cycle_modules()
-	return