avcodec/huffyuvdec: clear remainder of the array on end of input in decode_422_bitstream()

Fixes: msan_uninit-mem_7f909423efcd_9923_yuv_predmed.avi
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
