update() stale rebalance stats() object during pool expansion (#18882)

it is entirely possible that a rebalance process which was running
when it was asked to "stop" it failed to write its last statistics
to the disk.

After this a pool expansion can cause disruption and all S3 API
calls would fail at IsPoolRebalancing() function.

This PRs makes sure that we update rebalance.bin under such
conditions to avoid any runtime crashes.

cmd/background-newdisks-heal-ops.go

@@ -490,6 +490,10 @@ func healFreshDisk(ctx context.Context, z *erasureServerPools, endpoint Endpoint
 	}
 
 	for _, disk := range disks {
+		if disk == nil {
+			continue
+		}
+
 		t, err := loadHealingTracker(ctx, disk)
 		if err != nil {
 			if !errors.Is(err, errFileNotFound) {