avcodec/huffyuvdec: apply vertical filter in steps of 1 line for interlaced BGRA

Fixes out of array read
Fixes: signal_sigsegv_3287332_2301_cov_2994954934_huffyuv.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
