mm/pin_mem: add invalid check for pinmemory boot parameter

euleros inclusion
category: feature
bugzilla:https://gitee.com/openeuler/kernel/issues/I4UCEQ


CVE: NA

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When the pinmemory setting string is null, return directly
without setting pinmemory addr.

Signed-off-by: Jingxian He <hejingxian@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>