awful.layout.arrange(): Handle screen removal

If a screen is removed while a re-layout is pending, previously this code would
cause errors and problems. Since the screen is gone, there is nothing to arrange
anyway and we can just not do anything.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/layout/init.lua

@@ -162,6 +162,11 @@ function layout.arrange(screen)
     delayed_arrange[screen] = true
 
     timer.delayed_call(function()
+        if not screen.valid then
+            -- Screen was removed
+            delayed_arrange[screen] = nil
+            return
+        end
         if arrange_lock then return end
         arrange_lock = true