decoder/ffmpeg: use AVStream::duration

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -583,10 +583,7 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 	}
 
 	const SignedSongTime total_time =
-		format_context->duration != (int64_t)AV_NOPTS_VALUE
-		? SignedSongTime::FromScale<uint64_t>(format_context->duration,
-						      AV_TIME_BASE)
-		: SignedSongTime::Negative();
+		FromFfmpegTimeChecked(av_stream.duration, av_stream.time_base);
 
 	decoder_initialized(decoder, audio_format,
 			    input.IsSeekable(), total_time);