zombie00 test: fix for clang

CLang doesn't like explicit NULL pointer dereference:

> zombie00.c:52:5: error: indirection of non-volatile null pointer will be
> deleted, not trap [-Werror,-Wnull-dereference]
>                                 *(int *)NULL = 0;
>                                 ^~~~~~~~~~~~
> zombie00.c:52:5: note: consider using __builtin_trap() or qualifying
> pointer with 'volatile'
> 1 error generated.

Apparently, we need to add volatile to make it happy.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>