firstpass.c: fix an UBSAN/IOC error
firstpass.c: fix an UBSAN/IOC error

Change-Id: I579286e6741b689ae4281a35beb7b8f95c3ffce5

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