decoder/sidplay: fix playback speed with libsidplayfp

NEWS

 ver 0.19.20 (not yet released)
 * decoder
   - ffmpeg: ignore empty packets
+  - sidplay: fix playback speed with libsidplayfp
 
 ver 0.19.19 (2016/08/23)
 * decoder

src/decoder/plugins/SidplayDecoderPlugin.cxx

@@ -354,12 +354,19 @@ sidplay_file_decode(Decoder &decoder, Path path_fs)
 	DecoderCommand cmd;
 	do {
 		short buffer[4096];
-		size_t nbytes;
 
-		nbytes = player.play(buffer, ARRAY_SIZE(buffer));
-		if (nbytes == 0)
+		const auto result = player.play(buffer, ARRAY_SIZE(buffer));
+		if (result <= 0)
 			break;
 
+#ifdef HAVE_SIDPLAYFP
+		/* libsidplayfp returns the number of samples */
+		const size_t nbytes = result * sizeof(buffer[0]);
+#else
+		/* libsidplay2 returns the number of bytes */
+		const size_t nbytes = result;
+#endif
+
 		decoder_timestamp(decoder, (double)player.time() / timebase);
 
 		cmd = decoder_data(decoder, nullptr, buffer, nbytes, 0);