[release-v2.0] Prevent hang when dcrd connection is lost

The ordered shutdown of wallet goroutines before closing the websocket
connection introduced a new bug where the RPC syncer would hang if the dcrd
connection is lost before clean shutdown is signaled, due to a missing call to
cancel the context passed to Wallet.Run.

Backport of d3e2a05.

Author: jrick

chain/sync.go

@@ -759,13 +759,15 @@ func (s *Syncer) Run(ctx context.Context) (err error) {
 	log.Infof("Blockchain sync completed, wallet ready for general usage.")
 
 	g.Go(func() error {
+		var err error
 		select {
 		case <-ctx.Done():
-			walletCtxCancel()
-			return ctx.Err()
+			err = ctx.Err()
 		case <-wsClient.Done():
-			return wsClient.Err()
+			err = wsClient.Err()
 		}
+		walletCtxCancel()
+		return err
 	})
 	return g.Wait()
 }