Plugin crash when weave not running

[bboreham]

If someone (e.g. the weave script) tries to create a network when the weave container is not running, the plugin crashes after failing to obtain the default subnet:

DEBU: 2016/01/17 15:32:49.723144 RequestPool weavelocal   map[]
DEBU: 2016/01/17 15:32:49.723190 Getting IP for container weave
DEBU: 2016/01/17 15:32:49.724726 Networks:  map[host:{ 0   0   }]
DEBU: 2016/01/17 15:32:49.724773 weave GET to http://127.0.0.1:6784/ipinfo/defaultsubnet with map[]
DEBU: 2016/01/17 15:32:49.724919 RequestPool returning  <nil> Get http://127.0.0.1:6784/ipinfo/defaultsubnet: dial tcp 127.0.0.1:6784: getsockopt: connection refused
2016/01/17 15:32:49 http: panic serving @: runtime error: invalid memory address or nil pointer dereference
goroutine 16 [running]:
net/http.(*conn).serve.func1(0xc82006a370, 0x7f72976b4a30, 0xc820022198)
    /usr/local/go/src/net/http/server.go:1287 +0xb5
net.networkNumberAndMask(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /usr/local/go/src/net/ip.go:433 +0x42
net.(*IPNet).String(0x0, 0x0, 0x0)
    /usr/local/go/src/net/ip.go:483 +0x42
github.com/weaveworks/weave/plugin/ipam.(*ipam).RequestPool(0xc82009ce80, 0xc820107260, 0xa, 0x0, 0x0, 0x0, 0x0, 0x0, 0x40000000, 0x0, ...)
    /go/src/github.com/weaveworks/weave/plugin/ipam/driver.go:38 +0x4b8
github.com/weaveworks/weave/plugin/skel.(*listener).requestPool(0xc8200ac420, 0x7f72976b61e0, 0xc82006a630, 0xc82009a9a0)
    /go/src/github.com/weaveworks/weave/plugin/skel/listener.go:209 +0x13f
github.com/weaveworks/weave/plugin/skel.(*listener).(github.com/weaveworks/weave/plugin/skel.requestPool)-fm(0x7f72976b61e0, 0xc82006a630, 0xc82009a9a0)
    /go/src/github.com/weaveworks/weave/plugin/skel/listener.go:65 +0x3e
net/http.HandlerFunc.ServeHTTP(0xc8200bf9f0, 0x7f72976b61e0, 0xc82006a630, 0xc82009a9a0)

[bboreham]

To reproduce:

$ weave stop
$ weave launch-plugin
$ docker logs weave-plugin