vp8,calc_pframe_target_size: fix integer overflow
vp8,calc_pframe_target_size: fix integer overflow

this is similar to the fix for calc_iframe_target_size:
5f345a924 Avoid overflow in calc_iframe_target_size

Bug: chromium:1264506
Change-Id: I2f0e161cf9da59ca0724692d581f1594c8098ebb

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