net/neighbour: fix crash at dumping device-agnostic proxy entries

Proxy entries could have null pointer to net-device.

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Fixes: 84920c14 ("net: Allow ipv6 proxies and arp proxies be shown with iproute2")
Signed-off-by: David S. Miller <davem@davemloft.net>