ppc/spapr_events: fix potential NULL pointer dereference in rtas_event_log_dequeue

This fixes coverity issues 68911917:
        360
    CID 68911917: (NULL_RETURNS)
        361. dereference: Dereferencing "source", which is known to be
             "NULL".
        361        if (source->mask & event_mask) {
        362            break;
        363        }

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: PanNengyuan <pannengyuan@huawei.com>
Message-Id: <1574685291-38176-1-git-send-email-pannengyuan@huawei.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>