Fix ros dev panic

cmd/control/dev.go

@@ -2,10 +2,13 @@ package control
 
 import (
 	"fmt"
+
 	"github.com/codegangsta/cli"
 	"github.com/rancher/os/util"
 )
 
 func devAction(c *cli.Context) {
-	fmt.Println(util.ResolveDevice(c.Args()[0]))
+	if len(c.Args()) > 0 {
+		fmt.Println(util.ResolveDevice(c.Args()[0]))
+	}
 }