Fix C stack overflow upon portal destruction

Destroying a nether portal could cause a C stack overflow for
large portals when not using luajit. This patch defers removal of
the "next batch" of portal nodes to the next globalstep via
minetest.after. This has the additional bonus of a neat visual
effect depending on server conditions.

mods/ITEMS/mcl_portals/portal_nether.lua

@@ -93,14 +93,20 @@ local function destroy_nether_portal(pos)
 		return
 	end
 	if orientation == 0 then
-		check_remove({x = pos.x - 1, y = pos.y, z = pos.z}, 0)
-		check_remove({x = pos.x + 1, y = pos.y, z = pos.z}, 0)
+		minetest.after(0,function()
+			check_remove({x = pos.x - 1, y = pos.y, z = pos.z}, 0)
+			check_remove({x = pos.x + 1, y = pos.y, z = pos.z}, 0)
+		end)
 	else
-		check_remove({x = pos.x, y = pos.y, z = pos.z - 1}, 1)
-		check_remove({x = pos.x, y = pos.y, z = pos.z + 1}, 1)
+		minetest.after(0,function()
+			check_remove({x = pos.x, y = pos.y, z = pos.z - 1}, 1)
+			check_remove({x = pos.x, y = pos.y, z = pos.z + 1}, 1)
+		end)
 	end
-	check_remove({x = pos.x, y = pos.y - 1, z = pos.z})
-	check_remove({x = pos.x, y = pos.y + 1, z = pos.z})
+	minetest.after(0,function()
+		check_remove({x = pos.x, y = pos.y - 1, z = pos.z})
+		check_remove({x = pos.x, y = pos.y + 1, z = pos.z})
+	end)
 end
 
 minetest.register_node("mcl_portals:portal", {