This patch fixed a motion vector(MV) out of range bug, which was caused
by not restoring the original values of the MV min/max thresholds after
the sub8x8 full pixel motion search. It occurred rarely and only was seen
while encoding a 4k clip for 200 frames.

BUG=webm:1271

Change-Id: Ibc4e0de80846f297431923cef8a0c80fe8dcc6a5