server: fix wireToAddrmgrNetAddress data race.

[dnldd]

This fixes a data race for the network address passed into wireToAddrmgrNetAddress.

==================
WARNING: DATA RACE
Write at 0x00c01cef1f18 by goroutine 99:
  github.com/decred/dcrd/peer/v3.(*Peer).readRemoteVersionMsg()
      /Users/dnldd/Work/dcrd/peer/peer.go:1809 +0x214
  github.com/decred/dcrd/peer/v3.(*Peer).negotiateOutboundProtocol()
      /Users/dnldd/Work/dcrd/peer/peer.go:1944 +0x74
  github.com/decred/dcrd/peer/v3.(*Peer).start.func1()
      /Users/dnldd/Work/dcrd/peer/peer.go:1956 +0x7c

Previous read at 0x00c01cef1f18 by goroutine 97:
  main.wireToAddrmgrNetAddress()
      /Users/dnldd/Work/dcrd/server.go:616 +0x630
  main.(*server).outboundPeerConnected()
      /Users/dnldd/Work/dcrd/server.go:2243 +0x5c0
  main.(*server).outboundPeerConnected-fm()
      /Users/dnldd/Work/dcrd/server.go:2229 +0x58

Goroutine 99 (running) created at:
  github.com/decred/dcrd/peer/v3.(*Peer).start()
      /Users/dnldd/Work/dcrd/peer/peer.go:1952 +0x11c
  github.com/decred/dcrd/peer/v3.(*Peer).AssociateConnection.func1()
      /Users/dnldd/Work/dcrd/peer/peer.go:2013 +0x30

Goroutine 97 (finished) created at:
  github.com/decred/dcrd/connmgr/v3.(*ConnManager).connHandler()
      /Users/dnldd/Work/dcrd/connmgr/connmanager.go:334 +0x1080
==================