chan_websocket: Reset frame_queue_length to 0 after FLUSH_MEDIA

chan_websocket: Reset frame_queue_length to 0 after FLUSH_MEDIA

In the WebSocket channel driver, the FLUSH_MEDIA command clears all frames from
the queue but does not reset the frame_queue_length counter.

As a result, the driver incorrectly thinks the queue is full after flushing,
which prevents new multimedia frames from being sent, especially after multiple
flush commands.

This fix sets frame_queue_length to 0 after flushing, ensuring the queue state
is consistent with its actual content.

Fixes: #1304
(cherry picked from commit f03421b03b19305f06e34d3fa20dac7f21a59b82)