memcg: fix unsuitable null check after alloc memory

hulk inclusion
category: bugfix
bugzilla: 51815, https://gitee.com/openeuler/kernel/issues/I3IJ9I


CVE: NA

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Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>