awful.autofocus: Skip invalid screens

This code uses delayed calls to lazily update things. Thanks to this, it can try
to update a screen long after it was removed. Fix this by just doing nothing on
invalid screens.

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

lib/awful/autofocus.lua

@@ -18,6 +18,7 @@ local timer = require("gears.timer")
 --
 -- @param obj An object that should have a .screen property.
 local function check_focus(obj)
+    if not obj.screen.valid then return end
     -- When no visible client has the focus...
     if not client.focus or not client.focus:isvisible() then
         local c = aclient.focus.history.get(screen[obj.screen], 0, aclient.focus.filter)
@@ -39,7 +40,7 @@ end
 -- @param tag A tag object
 local function check_focus_tag(t)
     local s = t.screen
-    if not s then return end
+    if (not s) or (not s.valid) then return end
     s = screen[s]
     check_focus({ screen = s })
     if client.focus and screen[client.focus.screen] ~= s then