batman-adv: fix indirect hard_iface NULL dereference

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 149fc716558b65db4e1d697762e9317a12d856b1
("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>