avformat/iff: shrink packets to the initialized data

Fixes use of uninitialized data
Fixes: msan_uninit-mem_7f65b9788da6_388_24.iff
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
