scsi: hisi_sas: fix NULL deference when TMF timeouts

If a TMF timeouts (maybe due to unlikely scenario of an expander being
unplugged when TMF for remote device is active), when we eventually try
to free the slot, we crash as we dereference the slot's task, which has
already been released.

As a fix, add checks in the slot release code for a NULL task.

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