Be explicit about overflow in vpx_variance16x16_sse2.
Be explicit about overflow in vpx_variance16x16_sse2.

The product always fits in uint32_t, but the operands don't.

An optimizing compiler should generate the wraparound code.
(Verified with clang).

Change-Id: I25eb64df99152992bc898b8ccbb01d55c8d16e3c

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