make EarlyListener a struct, not an interface

interop/http09/http_test.go

@@ -36,8 +36,8 @@ var _ = Describe("HTTP 0.9 integration tests", func() {
 			defer close(done)
 			_ = server.ListenAndServe()
 		}()
-		var ln quic.EarlyListener
-		Eventually(func() quic.EarlyListener {
+		var ln *quic.EarlyListener
+		Eventually(func() *quic.EarlyListener {
 			server.mutex.Lock()
 			defer server.mutex.Unlock()
 			ln = server.listener

interop/http09/server.go

@@ -40,7 +40,7 @@ type Server struct {
 	QuicConfig *quic.Config
 
 	mutex    sync.Mutex
-	listener quic.EarlyListener
+	listener *quic.EarlyListener
 }
 
 // Close closes the server.