avcodec/motion_est_template: Fix undefined shifts in CHECK_MV_DIR()

Fixes: signal_sigsegv_2e64ee0_2762_cov_4170502227_missing_frames.avi

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