ipv6: fix crash over flow-based vxlan device

Similar check was added in ip_rcv but not in ipv6_rcv.

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff81734e0a>] ipv6_rcv+0xfa/0x500
Call Trace:
[<ffffffff816c9786>] ? ip_rcv+0x296/0x400
[<ffffffff817732d2>] ? packet_rcv+0x52/0x410
[<ffffffff8168e99f>] __netif_receive_skb_core+0x63f/0x9a0
[<ffffffffc02b34a0>] ? br_handle_frame_finish+0x580/0x580 [bridge]
[<ffffffff8109912c>] ? update_rq_clock.part.81+0x1c/0x40
[<ffffffff8168ed18>] __netif_receive_skb+0x18/0x60
[<ffffffff8168fa1f>] process_backlog+0x9f/0x150

Fixes: ee122c79 (vxlan: Flow based tunneling)
Signed-off-by: Wei-Chun Chao <weichunc@plumgrid.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>