ad_mpeg123: support in-stream format changes

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36461 b3059339-0415-0410-9bf9-f77b7e298cf2

Fixes playback of http://mpg123.org/test/44and22.mp3

Cherry-picked from MPlayer SVN rev. #36461, a patch by
Thomas Orgis, committed by by Reimar Döffinger.