Fix ubsan warning: vp9/encoder/vp9_encodeframe.c
Fix ubsan warning: vp9/encoder/vp9_encodeframe.c

BUG=webm:1219

Change-Id: I29b896c7f61abf871d7853b1d8200aa5a9f6a95c

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