return an error if we fail to parse external IP
```
I[06-08|11:51:57.234] Getting UPNP external address module=p2p
I[06-08|11:51:58.867] Got UPNP external address module=p2p address=
```
Fixes #1717
```
I[06-08|11:51:56.952] Starting multiAppConn module=proxy impl=multiAppConn
I[06-08|11:51:56.952] Starting localClient module=abci-client connection=query impl=localClient
I[06-08|11:51:56.952] Starting localClient module=abci-client connection=mempool impl=localClient
I[06-08|11:51:56.952] Starting localClient module=abci-client connection=consensus impl=localClient
I[06-08|11:51:56.952] ABCI Handshake module=consensus appHeight=0 appHash=
I[06-08|11:51:56.952] ABCI Replay Blocks module=consensus appHeight=0 storeHeight=0 stateHeight=0
I[06-08|11:51:57.053] Completed ABCI Handshake - Tendermint and App are synced module=consensus appHeight=0 appHash=
I[06-08|11:51:57.053] This node is a validator module=consensus addr=6816B5D9BAC32A3CDF07884D9D3D2650694C371D pubKey=PubKeyEd25519{27A40CD032DD2467342D0CF27C5EC92052D966FEC714B6CF2F3BF3146AFD0D51}
I[06-08|11:51:57.234] Starting Node module=main impl=Node
I[06-08|11:51:57.234] Starting EventBus module=events impl=EventBus
I[06-08|11:51:57.234] Local listener module=p2p ip=:: port=46656
I[06-08|11:51:57.234] Getting UPNP external address module=p2p
I[06-08|11:51:58.867] Got UPNP external address module=p2p address=
I[06-08|11:51:58.867] Starting DefaultListener module=p2p impl=Listener(@<nil>:46656)
I[06-08|11:51:58.867] P2P Node ID module=main ID=3629b516392e494ae717ac4c6a1ea7eb0fe421c3 file=/home/tpb/.tendermint/config/node_key.json
I[06-08|11:51:58.868] Add our address to book module=p2p book=/home/tpb/.tendermint/config/addrbook.json addr=null
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x89fb86]
goroutine 1 [running]:
github.com/tendermint/tendermint/p2p.(*NetAddress).String(0x0, 0xc96e24, 0x17)
/home/tpb/code/go/src/github.com/tendermint/tendermint/p2p/netaddress.go:171 +0x26
github.com/tendermint/tendermint/p2p/pex.(*addrBook).AddOurAddress(0xc420190620, 0x0)
/home/tpb/code/go/src/github.com/tendermint/tendermint/p2p/pex/addrbook.go:160 +0x116
github.com/tendermint/tendermint/node.(*Node).OnStart(0xc420286d00, 0xc4201b8010, 0xd)
/home/tpb/code/go/src/github.com/tendermint/tendermint/node/node.go:402 +0x547
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*BaseService).Start(0xc420286d00, 0xe51c40, 0xc42000bd40)
/home/tpb/code/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/service.go:130 +0x3bd
github.com/tendermint/tendermint/cmd/tendermint/commands.NewRunNodeCmd.func1(0xc42022e000, 0xc4200acdc0, 0x0, 0x1, 0x0, 0x0)
/home/tpb/code/go/src/github.com/tendermint/tendermint/cmd/tendermint/commands/run_node.go:58 +0xfe
github.com/tendermint/tendermint/vendor/github.com/spf13/cobra.(*Command).execute(0xc42022e000, 0xc4200acda0, 0x1, 0x1, 0xc42022e000, 0xc4200acda0)
/home/tpb/code/go/src/github.com/tendermint/tendermint/vendor/github.com/spf13/cobra/command.go:762 +0x468
github.com/tendermint/tendermint/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x1289280, 0xbbdda0, 0xc420015e01, 0xc4201bc640)
/home/tpb/code/go/src/github.com/tendermint/tendermint/vendor/github.com/spf13/cobra/command.go:852 +0x30a
github.com/tendermint/tendermint/vendor/github.com/spf13/cobra.(*Command).Execute(0x1289280, 0xc4201bc640, 0xc420015e98)
/home/tpb/code/go/src/github.com/tendermint/tendermint/vendor/github.com/spf13/cobra/command.go:800 +0x2b
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/cli.Executor.Execute(0x1289280, 0xde5798, 0x2, 0xc4200332c0)
/home/tpb/code/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/cli/setup.go:89 +0x4e
main.main()
/home/tpb/code/go/src/github.com/tendermint/tendermint/cmd/tendermint/main.go:45 +0x24d
```
cd3a240c9f