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 785ea11441
("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>

net/batman-adv/distributed-arp-table.c

@@ -440,7 +440,7 @@ static bool batadv_is_orig_node_eligible(struct batadv_dat_candidate *res,
 	/* this is an hash collision with the temporary selected node. Choose
 	 * the one with the lowest address
 	 */
-	if ((tmp_max == max) &&
+	if ((tmp_max == max) && max_orig_node &&
 	    (batadv_compare_eth(candidate->orig, max_orig_node->orig) > 0))
 		goto out;