libvpx

Avoid mv cost check for invalid mv values

The compound inter prediction could potentially run with initial
motion vectors of invalid value and check the mv_cost, which triggers
overheap read. This commit resolves this issue by forcing a motion
vector value check for compound inter modes of both superblock and
sub8x8 block sizes.

Change-Id: I4f4fc19ce83c8272782bc382f12c82a3f03212fc