flow_dissector: Check skb for VLAN only if skb specified.

Fixes a panic when calling eth_get_headlen(). Noticed on i40e driver.

Fixes: d5709f7ab776 ("flow_dissector: For stripped vlan, get vlan info from skb->vlan_tci")
Signed-off-by: Eric Garver <e@erig.me>
Reviewed-by: Jakub Sitnicki <jkbs@redhat.com>
Acked-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -247,12 +247,10 @@ ipv6:
 	case htons(ETH_P_8021Q): {
 		const struct vlan_hdr *vlan;
 
-		if (skb_vlan_tag_present(skb))
+		if (skb && skb_vlan_tag_present(skb))
 			proto = skb->protocol;
 
-		if (!skb_vlan_tag_present(skb) ||
-		    proto == cpu_to_be16(ETH_P_8021Q) ||
-		    proto == cpu_to_be16(ETH_P_8021AD)) {
+		if (eth_type_vlan(proto)) {
 			struct vlan_hdr _vlan;
 
 			vlan = __skb_header_pointer(skb, nhoff, sizeof(_vlan),