Currently, hex search couldn't guarantee the motion vector(MV)
found is within the limit of maximum MV. Therefore, very large
motion vectors resulted from big motion in the video could cause
encoding artifacts. This change adjusted hex search bounds
checking to make sure the resulted motion vector won't go out
of the range. James Berry, thank you for finding the bug.

Change-Id: If2c55edd9019e72444ad9b4b8688969eef610c55