Refactor kf_group_err in find_next_key_frame
Refactor kf_group_err in find_next_key_frame

Move the computation out of the while loop.

Change-Id: I00697e9a16d5d597c63e5d9895e4ae00efc7a2df

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