notification: Fix rendering during early screen creation.

If the problem happens early enough, it was possible that the
screen arrway wasn't initialized yet. In that case, the notification
would fail to render.

lib/naughty/core.lua

@@ -191,7 +191,9 @@ naughty.notifications = { suspended = { }, _expired = {{}} }
 
 naughty._active = {}
 
-screen.connect_for_each_screen(function(s)
+local function init_screen(s)
+    if naughty.notifications[s] then return end
+
     naughty.notifications[s] = {
         top_left = {},
         top_middle = {},
@@ -201,7 +203,9 @@ screen.connect_for_each_screen(function(s)
         bottom_right = {},
         middle = {},
     }
-end)
+end
+
+screen.connect_for_each_screen(init_screen)
 
 capi.screen.connect_signal("removed", function(scr)
     -- Allow the notifications to be moved to another screen.
@@ -646,6 +650,10 @@ local function register(notification, args)
 
     assert(s)
 
+    if not naughty.notifications[s] then
+        init_screen(get_screen(s))
+    end
+
     -- insert the notification to the table
     table.insert(naughty._active, notification)
     table.insert(naughty.notifications[s][notification.position], notification)