decoder/flac: handle unknown duration correctly

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().

NEWS

@@ -1,6 +1,7 @@
 ver 0.19.17 (not yet released)
 * decoder
   - flac: fix assertion failure while seeking
+  - flac: fix stream duration indicator
   - fix seek problems in several plugins
 * fix spurious seek error "Failed to allocate silence buffer"
 * replay gain: fix "replay_gain_handler mixer" setting

src/decoder/plugins/FlacCommon.hxx

@@ -55,9 +55,7 @@ struct flac_data : public FlacInput {
 	AudioFormat audio_format;
 
 	/**
-	 * The total number of frames in this song.  The decoder
-	 * plugin may initialize this attribute to override the value
-	 * provided by libFLAC (e.g. for sub songs from a CUE sheet).
+	 * The total number of frames in this song.  0 means unknown.
 	 */
 	FLAC__uint64 total_frames;
 

src/decoder/plugins/FlacDecoderPlugin.cxx

@@ -145,9 +145,10 @@ flac_decoder_initialize(struct flac_data *data, FLAC__StreamDecoder *sd,
 	if (data->initialized) {
 		/* done */
 
-		const auto duration2 =
-			SignedSongTime::FromScale<uint64_t>(data->total_frames,
-							    data->audio_format.sample_rate);
+		const auto duration2 = data->total_frames > 0
+			? SignedSongTime::FromScale<uint64_t>(data->total_frames,
+							      data->audio_format.sample_rate)
+			: SignedSongTime::Negative();
 
 		decoder_initialized(data->decoder, data->audio_format,
 				    data->input_stream.IsSeekable(),