libvpx

Fix an issue in motion vector prediction stage

The previous implementation stops motion vector prediction test when
the zero motion vector appears for the second time. This commit fixes
it by simply skipping the second time check on zero mv and continuing
on to next mv candidate.

It slightly improves stdhd in speed 2 by 0.06% on average. Most static
sequences are not affected. A few hard ones, like jet, ped, and riverbed
were improved by 0.1 - 0.2%.

Change-Id: Ia8d4e2ffb7136669e8ad1fb24ea6e8fdd6b9a3c1