naughty.legacy: Handle broken configs betters.

If a config with naughty.layouts.box crashes at startup and the
fallback config uses naughty.layouts.legaxy, it is possible their
will be some lookup for an object which isn't tracked by the legacy
module.

lib/naughty/layout/legacy.lua

@@ -134,7 +134,7 @@ local function get_offset(s, position, idx, width, height)
     local find_old_to_replace = function()
         for i = 1, idx-1 do
             local n = current_notifications[s][position][i]
-            if n.timeout > 0 then
+            if n and n.timeout > 0 then
                 return n
             end
         end