sched: fix dereference NULL pointers

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7EA1X


CVE: NA

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tg->auto_affinity is NULL if init_auto_affinity() failed.
So add checking for tg->auto_affinity before derefrence.

Fixes: 713cfd26 ("sched: Introduce smart grid scheduling strategy for cfs")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Reviewed-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Yipeng Zou <zouyipeng@huawei.com>