avformat/iss: check sscanf() return code

Fixes use of uninitialized data
Fixes: msan_uninit-mem_7f883205ce82_15_0001010100.iss
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
