Fixes to mp3 decode in subrepo. Filter out Xing/LAME info frames, rather than letting them go to the mp3 decoder as if they were audio frames, fixing the 1152-sample zero pad at start of some decoded mp3 files (distinct from decoder delay). Add MAD_BUFFER_GUARD padding at end of mp3 buffer, in order to ensure last frame is decoded successfully (otherwise the decoded audio is truncated). Both gleaned from madplay source code.