batman-adv: fix indirect hard_iface NULL dereference

If hard_iface is NULL and goto out is made batadv_hardif_free_ref()
doesn't check for NULL before dereferencing it to get to refcount.

Introduced in cb1c92ec
("batman-adv: add debugfs support to view multiif tables").
Reported-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Acked-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>

net/batman-adv/originator.c

@@ -1079,7 +1079,8 @@ int batadv_orig_hardif_seq_print_text(struct seq_file *seq, void *offset)
 	bat_priv->bat_algo_ops->bat_orig_print(bat_priv, seq, hard_iface);
 
 out:
-	batadv_hardif_free_ref(hard_iface);
+	if (hard_iface)
+		batadv_hardif_free_ref(hard_iface);
 	return 0;
 }