vxlan: fix oops when give unknown ifindex

If vxlan is created and the ifindex is passed; there are two cases which
are incorrectly handled by the existing code. The ifindex could be zero
(i.e. no device) or there could be no device with that ifindex.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>