network: add a nil check in (*TCPTransport).Close, prevent panic

You have to try to trigger that, but in the event of server shutdown before it
actually started listening Close will easily panic like this:

2020-03-05T11:48:08.660+0300    INFO    node started    {"blockHeight": 6458, "headerHeight": 15605}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x9d5f0a]

goroutine 1 [running]:
github.com/nspcc-dev/neo-go/pkg/network.(*TCPTransport).Close(0xc000fd7050)
        /home/rik/dev/neo-go/pkg/network/tcp_transport.go:78 +0x2a
github.com/nspcc-dev/neo-go/pkg/network.(*Server).Shutdown(0xc000182280)
        /home/rik/dev/neo-go/pkg/network/server.go:190 +0x189
github.com/nspcc-dev/neo-go/cli/server.startServer(0xc000200160, 0x0, 0x0)
        /home/rik/dev/neo-go/cli/server/server.go:367 +0x79e
github.com/urfave/cli.HandleAction(0xb84860, 0xccf240, 0xc000200160, 0xc0001c4500, 0x0)
        /home/rik/dev/neo-go/vendor/github.com/urfave/cli/app.go:490 +0xc8
github.com/urfave/cli.Command.Run(0xc9a160, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0xca407f, 0x10, 0x0, ...)
        /home/rik/dev/neo-go/vendor/github.com/urfave/cli/command.go:210 +0x996
github.com/urfave/cli.(*App).Run(0xc0001d64e0, 0xc0000ca000, 0x3, 0x3, 0x0, 0x0)
        /home/rik/dev/neo-go/vendor/github.com/urfave/cli/app.go:255 +0x6af
main.main()
        /home/rik/dev/neo-go/cli/main.go:25 +0x505

pkg/network/tcp_transport.go

@@ -75,7 +75,9 @@ func (t *TCPTransport) isCloseError(err error) bool {
 
 // Close implements the Transporter interface.
 func (t *TCPTransport) Close() {
+	if t.listener != nil {
 		t.listener.Close()
+	}
 }
 
 // Proto implements the Transporter interface.