batman-adv: Avoid nullptr dereference in bla after vlan_insert_tag

vlan_insert_tag can return NULL on errors. The bridge loop avoidance code
therefore has to check the return value of vlan_insert_tag for NULL before
it can safely operate on this pointer.

Fixes: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
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/bridge_loop_avoidance.c

@@ -418,9 +418,12 @@ static void batadv_bla_send_claim(struct batadv_priv *bat_priv, u8 *mac,
 		break;
 	}
 
-	if (vid & BATADV_VLAN_HAS_TAG)
+	if (vid & BATADV_VLAN_HAS_TAG) {
 		skb = vlan_insert_tag(skb, htons(ETH_P_8021Q),
 				      vid & VLAN_VID_MASK);
+		if (!skb)
+			goto out;
+	}
 
 	skb_reset_mac_header(skb);
 	skb->protocol = eth_type_trans(skb, soft_iface);