vxge: fix possible NULL dereference in vxge-traffic.c

If vpath is NULL then hldev is NULL also.

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Ram Vepa <ram.vepa@neterion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>