scsi: hisi_sas: fix NULL check in SMP abort task path

This patch adds a NULL check of task->lldd_task before freeing the
slot in SMP path.

This is to guard against the scenario of the slot being freed during
the from the preceding internal abort.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>