nvme-multipath: Early exit if no path is available

nvme_round_robin_path() should test if the return ns pointer is valid.
nvme_next_ns() will return a NULL pointer if there is no path left.

Fixes: 75c10e73 ("nvme-multipath: round-robin I/O policy")
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/multipath.c

@@ -221,7 +221,7 @@ static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head,
 	}
 
 	for (ns = nvme_next_ns(head, old);
-	     ns != old;
+	     ns && ns != old;
 	     ns = nvme_next_ns(head, ns)) {
 		if (nvme_path_is_disabled(ns))
 			continue;