sched: Avoid sched entity null pointer panic

hulk inclusion
category: bugfix
bugzilla: 51828, https://gitee.com/openeuler/kernel/issues/I4K96G


CVE: NA

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In some corner case, when we throttle the last group sched entity in rq,
kernel will be panic if there is no residual sched entities in rq. So we
add a protection to prevent this case.

Signed-off-by: Zheng Zucheng <zhengzucheng@huawei.com>
Reviewed-by: Chen Hui <judy.chenhui@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>