DecoderControl: use SignedSongTime for the song duration

src/decoder/DecoderAPI.cxx

@@ -59,7 +59,9 @@ decoder_initialized(Decoder &decoder,
 	dc.out_audio_format = getOutputAudioFormat(audio_format);
 
 	dc.seekable = seekable;
-	dc.total_time = total_time;
+	dc.total_time = total_time > 0
+		? SignedSongTime::FromS(total_time)
+		: SignedSongTime::Negative();
 
 	FormatDebug(decoder_domain, "audio_format=%s, seekable=%s",
 		    audio_format_to_string(dc.in_audio_format, &af_string),

src/decoder/DecoderControl.hxx

@@ -142,7 +142,7 @@ struct DecoderControl {
 	 */
 	SongTime end_time;
 
-	float total_time;
+	SignedSongTime total_time;
 
 	/** the #MusicChunk allocator */
 	MusicBuffer *buffer;