buffer.h: Only allow Init() to be called once.
buffer.h: Only allow Init() to be called once.

Change-Id: I041c8b6f314802833c5287a176dbfeec9461b08e

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