Tweak casts on vpx_sub_pixel_variance to avoid implicit overflow.
Tweak casts on vpx_sub_pixel_variance to avoid implicit overflow.

Change-Id: I481eb271b082fa3497b0283f37d9b4d1f6de270c

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