cgroup: fix dereference before null check

Coverity: 1230177 Dereference before null check

There may be a null pointer dereference, or else the comparison against
null is unnecessary.  In parse_task_cgroup: All paths that lead to this
null pointer comparison already dereference the pointer earlier
(CWE-476)

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>