decoder/mad: subtract libmad decoder delay from LAME encoder padding

Apparently, libmad not only inserts 529 samples of silence at the
beginning of the file, but also removes them at the end.

This solves the last piece of
https://github.com/MusicPlayerDaemon/MPD/issues/601

Closes https://github.com/MusicPlayerDaemon/MPD/issues/601

NEWS

@@ -1,6 +1,7 @@
 ver 0.21.12 (not yet released)
 * decoder
   - mad: update bit rate after seeking
+  - mad: fix several bugs preventing the plugin from decoding the last frame
   - opus: ignore case in replay gain tag names
   - opus, vorbis: decode the "end of stream" packet
 * output

src/decoder/plugins/MadDecoderPlugin.cxx

@@ -749,9 +749,17 @@ MadDecoder::DecodeFirstFrame(Tag *tag) noexcept
 		struct lame lame;
 		if (parse_lame(&lame, &ptr, &bitlen)) {
 			if (gapless_playback && input_stream.IsSeekable()) {
+				/* libmad inserts 529 samples of
+				   silence at the beginning and
+				   removes those 529 samples at the
+				   end */
 				drop_start_samples = lame.encoder_delay +
 				                           DECODERDELAY;
 				drop_end_samples = lame.encoder_padding;
+				if (drop_end_samples > DECODERDELAY)
+					drop_end_samples -= DECODERDELAY;
+				else
+					drop_end_samples = 0;
 			}
 
 			/* Album gain isn't currently used.  See comment in