Fix race condition

The download loop might finish and kill the worker goroutine before it's
done sending HAVE messages. Switching the order of these two lines
trivially fixes the issue (but it's still brittle)

p2p/p2p.go

@@ -173,8 +173,8 @@ func (t *Torrent) startDownloadWorker(peer peers.Peer, workQueue chan *pieceWork
 			continue
 		}
 
-		results <- &pieceResult{pw.index, buf}
 		c.have(pw.index)
+		results <- &pieceResult{pw.index, buf}
 	}
 }