avformat/ape: zero seektable&bittable and warn when they where only partially filled

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fcc198b365b_8417_sh3.ape
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
