avcodec/h264_mvpred: Fix undefined shifts in MAP_F2F

Fixes: asan_heap-oob_17301a3_2100_cov_3226131691_ff_add_pixels_clamped_mmx.m2ts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
