Crash on network device with no gateway

[bboreham]

Stack trace:

E1216 16:28:03.673776       1 runtime.go:78] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid m
emory address or nil pointer dereference)
goroutine 1 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic(0xdf6ca0, 0x176c510)
        /go/src/github.com/weaveworks/ignite/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:74 +0xa3
k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
        /go/src/github.com/weaveworks/ignite/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:48 +0x82
panic(0xdf6ca0, 0x176c510)
        /usr/local/go/src/runtime/panic.go:969 +0x166
github.com/weaveworks/ignite/pkg/container.takeAddress(0xc00059b938, 0xc0003a4f00, 0xc0006441ac, 0x4, 0x17b5d60, 0x203000)
        /go/src/github.com/weaveworks/ignite/pkg/container/network.go:200 +0x704
github.com/weaveworks/ignite/pkg/container.networkSetup(0xc00059bc70, 0xc00033ed68, 0xc0001dfb0c, 0xc000000180)
        /go/src/github.com/weaveworks/ignite/pkg/container/network.go:77 +0x1f7
github.com/weaveworks/ignite/pkg/container.SetupContainerNetworking.func1(0xc0001dfb40, 0x0, 0x0)
        /go/src/github.com/weaveworks/ignite/pkg/container/network.go:41 +0x2e

This was achieved with the following network config:

# cat /etc/cni/net.d/10-ignite.conflist 
{
 "cniVersion": "0.4.0",
 "name": "ignite-cni-bridge",
 "plugins": [
  {
   "name": "mynet",
   "type": "ipvlan",
   "master": "eth0",
   "ipam": {
    "type": "host-local",
    "subnet": "10.1.2.0/24"
   }
  },
  {
   "type": "portmap",
   "capabilities": {
    "portMappings": true
   }
  },
  {
   "type": "firewall"
  }
 ]
}

(turns out ipvlan won't work even with this crash fixed, but that's another story)