Nil pointer dereference on vpn client

[ersonp]

Describe the bug
There is a panic on starting vpn-client if vpn-client is started before the stun module is initilised.

Environment information:

• OS: e.g. Linux
• Platform: the equivalent of uname -smr or uname -smrv e.g. Darwin 17.4.0 x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Run ./skywire-visor -c skywire-config.json
2. Connect to a VPN server
3. See error

Actual behavior

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xb1db93]

goroutine 274 [running]:
github.com/skycoin/skywire/pkg/visor.getRouteSetupHooks.func1({0x3, 0xe6, 0xa4, 0x35, 0xb1, 0x6d, 0xe6, 0xac, 0x49, 0xd7, ...}, ...)
        /home/erson/Workspace/Go/skywire/pkg/visor/init.go:517 +0x413
github.com/skycoin/skywire/pkg/router.(*router).DialRoutes(0xc0005500b0, {0x1381bb0, 0xc00003a0b0}, {0x3, 0xe6, 0xa4, 0x35, 0xb1, 0x6d, 0xe6, ...}, ...)
        /home/erson/Workspace/Go/skywire/pkg/router/router.go:253 +0xd54
github.com/skycoin/skywire/pkg/app/appnet.(*SkywireNetworker).DialContext(0xc0005013e0, {0x1381bb0, 0xc00003a0b0}, {{0xc0028769f8, 0x6}, {0x3, 0xe6, 0xa4, 0x35, 0xb1, ...}, ...})
        /home/erson/Workspace/Go/skywire/pkg/app/appnet/skywire_networker.go:60 +0x16b
github.com/skycoin/skywire/pkg/app/appnet.DialContext({0x1381bb0, 0xc00003a0b0}, {{0xc0028769f8, 0x6}, {0x3, 0xe6, 0xa4, 0x35, 0xb1, 0x6d, ...}, ...})
        /home/erson/Workspace/Go/skywire/pkg/app/appnet/networker.go:82 +0x96
github.com/skycoin/skywire/pkg/app/appnet.Dial(...)
        /home/erson/Workspace/Go/skywire/pkg/app/appnet/networker.go:72
github.com/skycoin/skywire/pkg/app/appserver.(*RPCIngressGateway).Dial(0xc0025d4c60, 0xc0028d6000, 0xc002876a00)
        /home/erson/Workspace/Go/skywire/pkg/app/appserver/rpc_ingress_gateway.go:117 +0x208
reflect.Value.call({0xc000422180, 0xc00054afe8, 0x13}, {0xca471a, 0x4}, {0xc0027b3ef8, 0x3, 0x3})
        /usr/local/go/src/reflect/value.go:543 +0x814
reflect.Value.Call({0xc000422180, 0xc00054afe8, 0x0}, {0xc00273b6f8, 0x3, 0x3})
        /usr/local/go/src/reflect/value.go:339 +0xc5
net/rpc.(*service).call(0xc0025f6080, 0x0, 0x0, 0xc0025e42f0, 0xc00029fe00, 0xc00273b7d0, {0xbe3d40, 0xc0028d6000, 0xc00054ae70}, {0xb7b3e0, ...}, ...)
        /usr/local/go/src/net/rpc/server.go:377 +0x239
created by net/rpc.(*Server).ServeCodec
        /usr/local/go/src/net/rpc/server.go:474 +0x405

[ersonp]

==================
WARNING: DATA RACE
Write at 0x00c0003a2268 by goroutine 141:
  github.com/skycoin/skywire/pkg/visor.initStunClient()
      /home/erson/Workspace/Go/skywire/pkg/visor/init.go:299 +0xc4
  github.com/skycoin/skywire/pkg/visor.withInitCtx.func1()
      /home/erson/Workspace/Go/skywire/pkg/visor/init.go:1071 +0xce
  github.com/skycoin/skywire/pkg/visor/visorinit.(*Module).InitConcurrent()
      /home/erson/Workspace/Go/skywire/pkg/visor/visorinit/module.go:139 +0x4d6
  github.com/skycoin/skywire/pkg/visor/visorinit.(*Module).InitConcurrent·dwrap·4()
      /home/erson/Workspace/Go/skywire/pkg/visor/visorinit/module.go:117 +0x58

Previous read at 0x00c0003a2268 by goroutine 117:
  github.com/skycoin/skywire/pkg/visor.getRouteSetupHooks.func1()
      /home/erson/Workspace/Go/skywire/pkg/visor/init.go:517 +0x664
  github.com/skycoin/skywire/pkg/router.(*router).DialRoutes()
      /home/erson/Workspace/Go/skywire/pkg/router/router.go:253 +0x675
  github.com/skycoin/skywire/pkg/app/appnet.(*SkywireNetworker).DialContext()
      /home/erson/Workspace/Go/skywire/pkg/app/appnet/skywire_networker.go:60 +0x23c
  github.com/skycoin/skywire/pkg/app/appnet.DialContext()
      /home/erson/Workspace/Go/skywire/pkg/app/appnet/networker.go:82 +0xd7
  github.com/skycoin/skywire/pkg/app/appnet.Dial()
      /home/erson/Workspace/Go/skywire/pkg/app/appnet/networker.go:72 +0x2ad
  github.com/skycoin/skywire/pkg/app/appserver.(*RPCIngressGateway).Dial()
      /home/erson/Workspace/Go/skywire/pkg/app/appserver/rpc_ingress_gateway.go:117 +0x20c
  runtime.call32()
      /usr/local/go/src/runtime/asm_amd64.s:626 +0x48
  reflect.Value.Call()
      /usr/local/go/src/reflect/value.go:339 +0xd7
  net/rpc.(*service).call()
      /usr/local/go/src/net/rpc/server.go:377 +0x28c
  net/rpc.(*Server).ServeCodec·dwrap·4()
      /usr/local/go/src/net/rpc/server.go:474 +0x16b

Goroutine 141 (running) created at:
  github.com/skycoin/skywire/pkg/visor/visorinit.(*Module).InitConcurrent()
      /home/erson/Workspace/Go/skywire/pkg/visor/visorinit/module.go:117 +0x21b
  github.com/skycoin/skywire/pkg/visor.NewVisor()
      /home/erson/Workspace/Go/skywire/pkg/visor/visor.go:150 +0xbcf
  github.com/skycoin/skywire/cmd/skywire-visor/commands.runVisor()
      /home/erson/Workspace/Go/skywire/cmd/skywire-visor/commands/root.go:109 +0xb0e
  github.com/skycoin/skywire/cmd/skywire-visor/commands.runApp()
      /home/erson/Workspace/Go/skywire/cmd/skywire-visor/commands/nosystray.go:17 +0x44
  github.com/skycoin/skywire/cmd/skywire-visor/commands.glob..func2()
      /home/erson/Workspace/Go/skywire/cmd/skywire-visor/commands/root.go:60 +0x2d
  github.com/spf13/cobra.(*Command).execute()
      /home/erson/Workspace/Go/skywire/vendor/github.com/spf13/cobra/command.go:860 +0xaf3
  github.com/spf13/cobra.(*Command).ExecuteC()
      /home/erson/Workspace/Go/skywire/vendor/github.com/spf13/cobra/command.go:974 +0x5da
  github.com/spf13/cobra.(*Command).Execute()
      /home/erson/Workspace/Go/skywire/vendor/github.com/spf13/cobra/command.go:902 +0x126
  github.com/skycoin/skywire/cmd/skywire-visor/commands.Execute()
      /home/erson/Workspace/Go/skywire/cmd/skywire-visor/commands/root.go:141 +0x10f
  main.main()
      /home/erson/Workspace/Go/skywire/cmd/skywire-visor/skywire-visor.go:16 +0x35

Goroutine 117 (running) created at:
  net/rpc.(*Server).ServeCodec()
      /usr/local/go/src/net/rpc/server.go:474 +0x5f7
  net/rpc.(*Server).ServeConn()
      /usr/local/go/src/net/rpc/server.go:449 +0x63d
  github.com/skycoin/skywire/pkg/app/appserver.(*Proc).awaitConn·dwrap·2()
      /home/erson/Workspace/Go/skywire/pkg/app/appserver/proc.go:149 +0x58
==================