batman-adv: Fix invalid stack access in batadv_dat_select_candidates

batadv_dat_select_candidates provides an u32 to batadv_hash_dat but it
needs a batadv_dat_entry with at least ip and vid filled in.

Fixes: 3e26722bc9f2 ("batman-adv: make the Distributed ARP Table vlan aware")
Signed-off-by: Sven Eckelmann <sven@open-mesh.com>
Acked-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>