batman-adv: Avoid nullptr dereference in dat after vlan_insert_tag

vlan_insert_tag can return NULL on errors. The distributed arp table code
therefore has to check the return value of vlan_insert_tag for NULL before
it can safely operate on this pointer.

Fixes: be1db4f6615b ("batman-adv: make the Distributed ARP Table vlan aware")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

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

@@ -1009,9 +1009,12 @@ bool batadv_dat_snoop_outgoing_arp_request(struct batadv_priv *bat_priv,
 		if (!skb_new)
 			goto out;
 
-		if (vid & BATADV_VLAN_HAS_TAG)
+		if (vid & BATADV_VLAN_HAS_TAG) {
 			skb_new = vlan_insert_tag(skb_new, htons(ETH_P_8021Q),
 						  vid & VLAN_VID_MASK);
+			if (!skb_new)
+				goto out;
+		}
 
 		skb_reset_mac_header(skb_new);
 		skb_new->protocol = eth_type_trans(skb_new,
@@ -1089,9 +1092,12 @@ bool batadv_dat_snoop_incoming_arp_request(struct batadv_priv *bat_priv,
 	 */
 	skb_reset_mac_header(skb_new);
 
-	if (vid & BATADV_VLAN_HAS_TAG)
+	if (vid & BATADV_VLAN_HAS_TAG) {
 		skb_new = vlan_insert_tag(skb_new, htons(ETH_P_8021Q),
 					  vid & VLAN_VID_MASK);
+		if (!skb_new)
+			goto out;
+	}
 
 	/* To preserve backwards compatibility, the node has choose the outgoing
 	 * format based on the incoming request packet type. The assumption is