This improves motion estimation and avoids using uninitialized data
for resolutions that aren't a multiple of 16.

Prior to d2a25c40, the edges used to be initialized so that encoding
was deterministic, but after that commit it started using uninitialized
data (for non multiple of 16 resolutions).

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>