Use default channel layout if zero

libav 11 reports an invalid channel layout for mp3 files. This is a
work-around. The problem is fixed with libav 11.1.

src/player.c

@@ -216,6 +216,13 @@ static bool openFilter (player_t * const player) {
 
 	/* abuffer */
 	AVRational time_base = player->st->time_base;
+
+	/* Workaround for a bug in libav-11, which reports an invalid channel
+	 * layout mp3 files */
+	if (cctx->channel_layout == 0) {
+		cctx->channel_layout = av_get_default_channel_layout (cctx->channels);
+	}
+
 	snprintf (strbuf, sizeof (strbuf),
 			"time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=0x%"PRIx64, 
 			time_base.num, time_base.den, cctx->sample_rate,