naughty: fix a bug on timeout when popup was destroyed by hover_timeout

Signed-off-by: koniu <gkusnierz@gmail.com>

lib/naughty.lua.in

@@ -152,7 +152,7 @@ end
 -- @param notification Notification object to be destroyed
 -- @return True if the popup was successfully destroyed, nil otherwise
 function destroy(notification)
-    if notification then
+    if notification and notification.box.screen then
         local scr = notification.box.screen
         table.remove(notifications[notification.box.screen][notification.position], notification.idx)
         hooks.timer.unregister(notification.die)