remove invalid assertion - #6634

remove invalid assertion - #6634

There is a highly unlikely case where the render thread can have zero
requests in the queue, but it is not supposed to be terminated.

 1) WaveView::queue_get_image();
wake up thread, *but* the thread does not start yet
 2) WaveView::cancel_my_render_request();
and now the thread starts.

1,2 are initiated by user actions from the GUI thread and are normally
orders of magnitude slower than scheduler-thread wakeup.