scsi: hisi_sas: add null check before indirect pointer dereference

Add null check before indirectly dereferencing pointer task->lldd_task
in statement u32 tag = slot->idx;

Addresses-Coverity-ID: 1373843
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>