avformat/jvdec: clear packet padding after downsize

Fixes: use of uninitialized memeory
Fixes: msan_uninit-mem_7fbf26b5fefe_5981_intro.jv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
