vp8/error_concealment: remove shift of negative value
vp8/error_concealment: remove shift of negative value

fixes:
shifting a negative signed value is undefined [-Wshift-negative-value]

Change-Id: I3a55f2dac7c51c0e264d40081ffce98e2abacb89

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