Impact: avoid accessing NULL tg.css->cgroup

In commit 0a0db8f5, I removed checking
NULL tg.css->cgroup, but I realized I was wrong when I found reading
/proc/sched_debug can race with cgroup_create().
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>