avcodec/iff: ensure that runs with insufficient input dont leave uninitialized bytes in the output

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fa0dea15eae_8988_test.iff
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
