player: don't play empty chunks

An empty chunk may happen when it only contains a tag, but no PCM
data.  Don't call playAudio() then.

src/player_thread.c

@@ -236,6 +236,9 @@ playChunk(struct song *song, struct music_chunk *chunk,
 		}
 	}
 
+	if (chunk->length == 0)
+		return 0;
+
 	pcm_volume(chunk->data, chunk->length,
 		   format, pc.softwareVolume);