variance aq: Fix a variance calculation overflow bug.
variance aq: Fix a variance calculation overflow bug.

This is an actual overflow where the result of the calculation is
materially changed, not just a negative value that is stored in an
unsigned.

Caught with fsanitize=integer on the VP9/AqSegmentTest.TestNoMisMatchAQ2/1 test.

Change-Id: I514b0ef4ae7ad50e3e08c0079aa204d59fa679aa

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