avformat/oggparsevorbis: check packet size before reading new_len from it

Fixes out of array read
Fixes: 34260c7981118fb38fba61809bf4dd5a-asan_heap-oob_93b923_1508_cov_951051643_DivX640x480_oggvorbis.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>