scsi: hisi_sas: Check before using pointer variables

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8IL6H


CVE: NA

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In commit 4b329abc ("scsi: hisi_sas: Move slot variable definition in
hisi_sas_abort_task()"), we move the variables slot to the function head.
However, the variable slot may be NULL, we should check it in each branch.

Fixes: 4b329abc ("scsi: hisi_sas: Move slot variable definition in hisi_sas_abort_task()")
Signed-off-by: Yihang Li <liyihang9@huawei.com>
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: xiabing <xiabing12@h-partners.com>