misc-fix: clamp MVs before find_ref_mvs()

This commit merge the fix that adds clamping to motion vectors before
they are added to list of MVs in find_ref_mvs().

Related tracking issue:
https://bugs.chromium.org/p/webm/issues/detail?id=1012
Original libvpx commit:
293e20df9128444a21b8ac483ee88e96386861c2

Change-Id: I6916a321f374f521187aae96d87bcfd2954db629