avformat/isom: check avio_read() result

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f7aa4151661_4885_dmbts3.ts
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
