Don't try to read freezer.state from the current directory

If we try to pause a container on the system without freezer cgroups,
we can found that runc tries to open ./freezer.state. It is obviously wrong.

$ ./runc pause test
no such directory for freezer.state

$ echo FROZEN > freezer.state
$ ./runc pause test
container not running or created: paused

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

libcontainer/container_linux.go

@@ -1284,7 +1284,12 @@ func (c *linuxContainer) runType() (Status, error) {
 }
 
 func (c *linuxContainer) isPaused() (bool, error) {
-	data, err := ioutil.ReadFile(filepath.Join(c.cgroupManager.GetPaths()["freezer"], "freezer.state"))
+	fcg := c.cgroupManager.GetPaths()["freezer"]
+	if fcg == "" {
+		// A container doesn't have a freezer cgroup
+		return false, nil
+	}
+	data, err := ioutil.ReadFile(filepath.Join(fcg, "freezer.state"))
 	if err != nil {
 		// If freezer cgroup is not mounted, the container would just be not paused.
 		if os.IsNotExist(err) {