avcodec/motion_est: use 2x8x8 for interlaced qpel

Fixes out of array read
Fixes Ticket4121

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b50e003e1cb6a215df44ffa3354603bf600b4aa3)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
1 file changed