decoder/ffmpeg: ignore empty packets

An empty packet would be a command for avcodec_send_packet() to
finalize the codec.

Fixes https://bugs.musicpd.org/view.php?id=4588

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -643,7 +643,7 @@ FfmpegDecode(Decoder &decoder, InputStream &input,
 			/* end of file */
 			break;
 
-		if (packet.stream_index == audio_stream) {
+		if (packet.size > 0 && packet.stream_index == audio_stream) {
 			cmd = ffmpeg_send_packet(decoder, input,
 						 packet,
 						 *codec_context,