cgroup: only reset directory when dirnew is valid

In the case where we don't rewrite anything, dirnew is never set, so we
shouldn't reset anything, because we didn't do any work.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cgroup.c

@@ -1743,8 +1743,10 @@ static int rewrite_cgsets(CgroupEntry *cge, char **controllers, int n_controller
 		}
 	}
 
-	xfree(dir);
-	*dir_name = dirnew;
+	if (dirnew) {
+		xfree(dir);
+		*dir_name = dirnew;
+	}
 	return 0;
 }