peer: when failing link, depend on server wg, not peer

In this commit, we fix a recently introduced bug. The issue is that
while we're failing the link, the peer we're attempting to force close
on may disconnect. As a result, if the peerTerminationWatcher exits
before we can add to the wait group (it's waiting on that), then we'll
run into a panic as we're attempting to increment the wait group while
another goroutine is calling wait.

The fix is to first check that the server isn't shutting down, and then
use the server's wait group rather than the peer to synchronize
goroutines.

Fixes #1285.

peer.go

@@ -438,14 +438,23 @@ func (p *peer) addLink(chanPoint *wire.OutPoint,
 		shortChanID lnwire.ShortChannelID,
 		linkErr htlcswitch.LinkFailureError) {
 
+		select {
+		// If the server is already exiting, then none of the actions
+		// below can finish exiting, so we'll exit early as well.
+		case <-p.server.quit:
+			return
+
+		default:
+		}
+
 		// The link has notified us about a failure. We launch a go
 		// routine to stop the link, disconnect the peer and optionally
 		// force close the channel. We must launch a goroutine since we
 		// must let OnChannelFailure return in order for the link to
 		// completely stop in the call to RemoveLink.
-		p.wg.Add(1)
+		p.server.wg.Add(1)
 		go func() {
-			defer p.wg.Done()
+			defer p.server.wg.Done()
 
 			// We begin by removing the link from the switch, such
 			// that it won't be attempted used for any more