vpx_mem: add basic size check
vpx_mem: add basic size check

set a max allocable size to prevent overflows in 32-bit and extremely
large allocation attempts in 64-bit. this could be amended to allow size
or num parameters to be 64-bits with the correct size being used at each
call site.

BUG=webm:819

Change-Id: Ia81004d6c4279680714c4488b4f6cf287ab396a5

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