avcodec/iff: warn about truncated input to decode_byterun() and clear remaining output

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fe205e395ed_8988_test.iff
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
