a quick fix to #15626

Signed-off-by: Shijiang Wei <mountkin@gmail.com>

daemon/daemon.go

@@ -979,7 +979,7 @@ func getDefaultRouteMtu() (int, error) {
 		return 0, err
 	}
 	for _, r := range routes {
-		if r.Default {
+		if r.Default && r.Iface != nil {
 			return r.Iface.MTU, nil
 		}
 	}