[timeline] Set the n_markers out paramater

Bug 1464 - clutter_timeline_list_markers should set n_markers=0 when
           there are no markers

If no markers are registered and we bail out we should also set the
n_markers out parameter to 0 so that code checking the number of
markers will still work.

clutter/clutter-timeline.c

@@ -1639,6 +1639,9 @@ clutter_timeline_list_markers (ClutterTimeline *timeline,
   if (G_UNLIKELY (priv->markers_by_name == NULL ||
                   priv->markers_by_frame == NULL))
     {
+      if (n_markers)
+        *n_markers = 0;
+
       return NULL;
     }