batman-adv: Fix NULL pointer dereference in DAT hash collision avoidance

An entry in DAT with the hashed position of 0 can cause a NULL pointer
dereference when the first entry is checked by batadv_choose_next_candidate.
This first candidate automatically has the max value of 0 and the max_orig_node
of NULL. Not checking max_orig_node for NULL in batadv_is_orig_node_eligible
will lead to a NULL pointer dereference when checking for the lowest address.

This problem was added in 785ea114


("batman-adv: Distributed ARP Table - create DHT helper functions").

Signed-off-by: Pau Koning <paukoning@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>