move (av_)find_stream_index() to utils.c as its usefull outside mpeg.c
assert checking enabled, to find bugs quicker, should obviously be disabled later
(av_)add_index_entry() inserts new entries so that the list stays ordered and updates entries if already in it
(av_)index_search_timestamp() cleanup (kill ugly goto) and shorter


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@2697 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b