libs/web: fix possible dispatcher crash

libs/web/luasrc/dispatcher.lua

@@ -93,16 +93,19 @@ end
 -- @return		Ordered table of child node names
 function node_childs(node)
 	local rv = { }
+	if node then
 		local k, v
 		for k, v in util.spairs(node.nodes,
 			function(a, b)
-	    return (node.nodes[a].order or 100) < (node.nodes[b].order or 100)
+				return (node.nodes[a].order or 100)
+				     < (node.nodes[b].order or 100)
 			end)
 		do
 			if node_visible(v) then
 				rv[#rv+1] = k
 			end
 		end
+	end
 	return rv
 end