mad: helpfully bodge the last buffer to let mad decode the last frame

mad: helpfully bodge the last buffer to let mad decode the last frame

If http://www.mars.org/mailman/public/mad-dev/2001-May/000262.html is
to be believed, the last buffer must be followed by a number of 0 bytes
in order for the last frame to be decoded (at least in some cases).
Doing so seems to work here, fixing a missing 1152 samples when using
mp3parse before mad (not using mp3parse would yield the correct amount
of samples, if there's extra non-MP3 data after (eg, tag data)).