avcodec/wnv1: clear padding area of rbuf

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7faa48586020_2145_MAILTEST.AVI
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
