avcodec/vc1: fix DIFF2/NORM2 with width<=16

Fixes read of uninitialized memory
Fixes msan_uninit-mem_7f785da000e8_585_480i30__codec_WVC1__mode_2__framerate_29.970__type_2__preproc_17.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
