libvpx

added clamp for 2nd motion vector

The commit added a clamp to the 2nd motion vector used in compound
prediction to insure mv within UMV borders. The clamp is similar to
that of the first motion vector except that No SPLITMV is ever used
for the 2nd motion vector.

Change-Id: I26dd63c304bd66b2e03a083749cc98c641667116