vp8_update_rate_correction_factors: fix integer overflow
vp8_update_rate_correction_factors: fix integer overflow

the intermediate value in the correction_factor calculation may exceed
integer bounds

Bug: b/189602769
Change-Id: I75726b12f3095663911d78333f3ea26eb6dee21e

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx