Resolve -Wshorten-64-to-32 warnings in prob.h.
Resolve -Wshorten-64-to-32 warnings in prob.h.

Change-Id: I1244ee908d81467f0fc8a8fce979fc8077a325b4

ShiftMediaProject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx