player/Thread: initialize MusicChunk::bit_rate in SendSilence()

This attribute is not particularly important, but it was
uninitialized.

src/player/Thread.cxx

@@ -558,6 +558,7 @@ Player::SendSilence()
 	   partial frames */
 	unsigned num_frames = sizeof(chunk->data) / frame_size;
 
+	chunk->bit_rate = 0;
 	chunk->time = SignedSongTime::Negative(); /* undefined time stamp */
 	chunk->length = num_frames * frame_size;
 	PcmSilence({chunk->data, chunk->length}, play_audio_format.format);