arm: kaslr: Fix memtop calculate, when there is no memory top info, we can't use zero instead it.

hulk inclusion
commit 88bf5c03832d56c68fac61e4ae97158b3332bd63
category: feature
feature: ARM kaslr support
bugzilla: 47952
CVE: NA

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Fix memtop calculate, when there is no memory top info,
we can't use zero instead it.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Cui GaoSheng <cuigaosheng1@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>