Fix a signed overflow in vp9 motion cost.
Fix a signed overflow in vp9 motion cost.

Change-Id: I5975e3aede62202d8ee6ced33889350c0a56554a

webmproject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mbebenita/aom,mwgoldsmith/vpx,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mbebenita/aom,ittiamvpx/libvpx-1,luctrudeau/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,smarter/aom,mbebenita/aom,ittiamvpx/libvpx-1,Topopiccione/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,GrokImageCompression/aom,luctrudeau/aom,Topopiccione/libvpx,mbebenita/aom,ittiamvpx/libvpx-1,smarter/aom,mbebenita/aom,ittiamvpx/libvpx-1,smarter/aom,Topopiccione/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,smarter/aom,ShiftMediaProject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,webmproject/libvpx,mbebenita/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,ShiftMediaProject/libvpx,mbebenita/aom,mwgoldsmith/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,webmproject/libvpx,GrokImageCompression/aom,Topopiccione/libvpx