flow_dissector: Check skb for VLAN only if skb specified.

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

Fixes: d5709f7a

 ("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>