Fix race in consumer create (#4377)

This fixes the race condition in consumer create API by adding a missing
return statement, probably introduced while solving conflicts.

Signed-off-by: Tomasz Pietrek <tomasz@nats.io>

server/jetstream_cluster.go

@@ -6897,6 +6897,7 @@ func (s *Server) jsClusteredConsumerRequest(ci *ClientInfo, acc *Account, subjec
 			if action == ActionCreate && !reflect.DeepEqual(cfg, ca.Config) {
 				resp.Error = NewJSConsumerAlreadyExistsError()
 				s.sendAPIErrResponse(ci, acc, subject, reply, string(rmsg), s.jsonResponse(&resp))
+				return
 			}
 			// Do quick sanity check on new cfg to prevent here if possible.
 			if err := acc.checkNewConsumerConfig(ca.Config, cfg); err != nil {