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