avformat/oggparsetheora: zero extradata padding area

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f66fa0ac09d_9197_theora_a4_v6_k250_s0.ogg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
