Correct the allocation size for ssim_vars
Correct the allocation size for ssim_vars

Ssim_vars is used to accumulate stats based 4x4 pixel blocks, this
commit changes the allocations size to be based on mi_rows and mi_cols
to avoid out-of-bound memory access for larger size videos. The hard
coded 720x480 can only work for image size up to 2880x1920.

Change-Id: Id9d07f3f777385b448ac88a6034b7472e4cf3c79

VTCSecureLLC/libvpx,luctrudeau/aom,felipebetancur/libvpx,shacklettbp/aom,mbebenita/aom,luctrudeau/aom,ittiamvpx/libvpx-1,smarter/aom,shacklettbp/aom,jmvalin/aom,mbebenita/aom,zofuthan/libvpx,webmproject/libvpx,zofuthan/libvpx,Topopiccione/libvpx,felipebetancur/libvpx,liqianggao/libvpx,mwgoldsmith/vpx,thdav/aom,VTCSecureLLC/libvpx,smarter/aom,ittiamvpx/libvpx-1,mbebenita/aom,smarter/aom,openpeer/libvpx_new,felipebetancur/libvpx,openpeer/libvpx_new,GrokImageCompression/aom,Topopiccione/libvpx,zofuthan/libvpx,thdav/aom,luctrudeau/aom,webmproject/libvpx,Topopiccione/libvpx,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,GrokImageCompression/aom,thdav/aom,openpeer/libvpx_new,felipebetancur/libvpx,luctrudeau/aom,mbebenita/aom,running770/libvpx,luctrudeau/aom,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,liqianggao/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,shyamalschandra/libvpx,GrokImageCompression/aom,thdav/aom,mwgoldsmith/vpx,running770/libvpx,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/libvpx,shyamalschandra/libvpx,ShiftMediaProject/libvpx,running770/libvpx,thdav/aom,ittiamvpx/libvpx-1,felipebetancur/libvpx,running770/libvpx,jmvalin/aom,shyamalschandra/libvpx,openpeer/libvpx_new,felipebetancur/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,ittiamvpx/libvpx-1,zofuthan/libvpx,jmvalin/aom,smarter/aom,thdav/aom,mwgoldsmith/libvpx,zofuthan/libvpx,webmproject/libvpx,webmproject/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,openpeer/libvpx_new,mbebenita/aom,mbebenita/aom,smarter/aom,liqianggao/libvpx,mwgoldsmith/vpx,jmvalin/aom,zofuthan/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ittiamvpx/libvpx-1,shacklettbp/aom,VTCSecureLLC/libvpx,openpeer/libvpx_new,jmvalin/aom,Topopiccione/libvpx,running770/libvpx,liqianggao/libvpx,shyamalschandra/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,shacklettbp/aom,smarter/aom,jmvalin/aom,shacklettbp/aom,Topopiccione/libvpx,liqianggao/libvpx,running770/libvpx,GrokImageCompression/aom,shacklettbp/aom,mwgoldsmith/libvpx,shyamalschandra/libvpx,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,mbebenita/aom,VTCSecureLLC/libvpx,mbebenita/aom,liqianggao/libvpx,shyamalschandra/libvpx