decoder/plugins/FfmpegIo: return AVERROR_EOF at end of file

This part of the AVIOContext API is not documented :-(

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1448

src/decoder/plugins/FfmpegIo.cxx

@@ -35,7 +35,11 @@ AvioStream::~AvioStream()
 inline int
 AvioStream::Read(void *dest, int size)
 {
-	return decoder_read(client, input, dest, size);
+	const auto nbytes = decoder_read(client, input, dest, size);
+	if (nbytes == 0)
+		return AVERROR_EOF;
+
+	return nbytes;
 }
 
 inline int64_t