crash if getNativeBuffer() called on NULL GraphicBuffer

If getNativeBuffer() is called on a NULL GraphicBuffer the
static_cast of this from GraphicBuffer* to ANativeWindowBuffer*
will return a small pointer like (ANativeWindowBuffer*)0x10.
This value can propagate past NULL checks until it causes a crash
far away from the original NULL pointer.  Crash immediately
instead.

Change-Id: Id614b9eb1484108b3c3c733545309844c4b87532