batadv_softif_vlan_get() may return NULL which has to be verified
by the caller.

Fixes: 35df3b29 ("batman-adv: fix TT VLAN inconsistency on VLAN re-add")
Reported-by: NRyan Thompson <ryan@eero.com>
Signed-off-by: NMarek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: NAntonio Quartulli <antonio@meshcoding.com>