mvsas: Fix NULL pointer dereference in mvs_slot_task_free

mvsas: Fix NULL pointer dereference in mvs_slot_task_free

When pci_pool_alloc fails in mvs_task_prep then task->lldd_task stays
NULL but it's later used in mvs_abort_task as slot which is passed
to mvs_slot_task_free causing NULL pointer dereference.

Just return from mvs_slot_task_free when passed with NULL slot.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=101891
Signed-off-by: Dāvis Mosāns <[email protected]>
Reviewed-by: Tomas Henzl <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Cc: [email protected]
Signed-off-by: James Bottomley <[email protected]>