prevent nil addr

Error:

```
Error: runtime error: invalid memoryaddress or nil pointer dereference\nStack:
goroutine 549 [running]:\nruntime/debug.Stack(0x0, 0x0,
0x0)\n\t/usr/local/go/src/runtime/debug/stack.go:24
+0x80\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*MConnection)._recover(0xc821723b00)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/connection.go:173
+0x53\npanic(0xbe1500, 0xc820012080)\n\t/usr/local/go/src/runtime/panic.go:443
+0x4e9\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*NetAddress).Valid(0x0,
0x0)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/netaddress.go:125
+0x1c\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*NetAddress).Routable(0x0,
0xc8217bb740)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/netaddress.go:117
+0x25\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*AddrBook).addAddress(0xc820108380,
0x0,
0xc821739590)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/addrbook.go:524
+0x45\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*AddrBook).AddAddress(0xc820108380,
0x0,
0xc821739590)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/addrbook.go:160
+0x286\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*PEXReactor).Receive(0xc82000be60,
0xc820149f00, 0xc8218163f0, 0xc82184e000, 0x5b,
0x1000)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/pex_reactor.go:109
+0x457\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.newPeer.func1(0xc82011d500,
0xc82184e000, 0x5b,
0x1000)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/peer.go:58
+0x202\ngithub.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*MConnection).recvRoutine(0xc821723b00)\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/connection.go:439
+0x1177\ncreated by
github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p.(*MConnection).OnStart\n\t/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-p2p/connection.go:138
+0x1a1\n
```

pex_reactor.go

@@ -112,7 +112,9 @@ func (pexR *PEXReactor) Receive(chID byte, src *Peer, msgBytes []byte) {
 		// (We don't want to get spammed with bad peers)
 		srcAddr := src.Connection().RemoteAddress
 		for _, addr := range msg.Addrs {
-			pexR.book.AddAddress(addr, srcAddr)
+			if addr != nil {
+				pexR.book.AddAddress(addr, srcAddr)
+			}
 		}
 	default:
 		log.Warn(Fmt("Unknown message type %v", reflect.TypeOf(msg)))