Fix Theora video playback without a Vorbis stream

- prevent audio resampler errors when number of channels is 0,
- don't check for 'audio_done' when there is no audio data.

drivers/theora/video_stream_theora.cpp

@@ -513,7 +513,7 @@ void VideoStreamPlaybackTheora::update(float p_delta) {
 	}
 
 	bool frame_done=false;
-	bool audio_done=false;
+	bool audio_done=!vorbis_p;
 
 	bool theora_done=false;