VP9 HBD: Fix integer overflow problem in variance calc.
VP9 HBD: Fix integer overflow problem in variance calc.

BUG=webm:1534
Change-Id: I535ac48e3dd2454cc7088c4f9a1e08ea74107da6

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx