criu: pstree_switch_state -- Fix nil deref on error path

We may enter with @root_item = NULL in case of error
handling, so simply bail out.
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/seize.c

@@ -463,6 +463,9 @@ void pstree_switch_state(struct pstree_item *root_item, int st)
 {
 	struct pstree_item *item = root_item;
 
+	if (!root_item)
+		return;
+
 	if (st != TASK_DEAD)
 		freezer_restore_state();