scsi: hisi_sas: Check before using pointer variables

scsi: hisi_sas: Check before using pointer variables

In commit 4b329abc9180 ("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: 4b329abc9180 ("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>
Link: https://lore.kernel.org/r/1702525516-51258-4-git-send-email-chenxiang66@hisilicon.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>