queue2: Only update current level if we already downloaded a range

Otherwise queue->level is NULL and dereferencing that is not a good
idea in general.

https://bugzilla.gnome.org/show_bug.cgi?id=707648

plugins/elements/gstqueue2.c

@@ -877,6 +877,7 @@ update_buffering (GstQueue2 * queue)
   gboolean post = FALSE;
 
   /* Ensure the variables used to calculate buffering state are up-to-date. */
+  if (queue->current)
     update_cur_level (queue, queue->current);
   update_in_rates (queue);