We may reach NULL dereference oops if kmalloc failed.
Prevent it with explicit BUG_ON.

[ Impact: more controlled assert in 'impossible' scenario ]
Signed-off-by: NCyrill Gorcunov <gorcunov@openvz.org>
Acked-by: NJack Steiner <steiner@sgi.com>
LKML-Reference: <20090501202511.GE4633@lenovo>
Signed-off-by: NIngo Molnar <mingo@elte.hu>