avformat/adxdec: dont return random data on EOF

Fixes use of uninitialized memory
Partly fixes; msan_uninit-mem_7fb7d24780d0_2744_R03T.CAK
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
