set missing logger on switch

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x882cec]

goroutine 328 [running]:
github.com/tendermint/tendermint/p2p.(*Switch).DialPeerWithAddress(0xc42000a500, 0xc4202088d0, 0xc420403500, 0x0, 0x0, 0x0)
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/switch.go:324 +0x2fc
github.com/tendermint/tendermint/p2p.(*PEXReactor).ensurePeers.func1(0xc4201663f0, 0xc4202088d0)
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/pex_reactor.go:280 +0x3e
created by github.com/tendermint/tendermint/p2p.(*PEXReactor).ensurePeers
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/pex_reactor.go:284 +0x5d4
```

p2p/pex_reactor_test.go

@@ -75,6 +75,8 @@ func TestPEXReactorRunning(t *testing.T) {
 	// create switches
 	for i := 0; i < N; i++ {
 		switches[i] = makeSwitch(config, i, "127.0.0.1", "123.123.123", func(i int, sw *Switch) *Switch {
+			sw.SetLogger(log.TestingLogger().With("switch", i))
+
 			r := NewPEXReactor(book)
 			r.SetLogger(log.TestingLogger())
 			r.SetEnsurePeersPeriod(250 * time.Millisecond)