[v1.39.x_conn_leak] server: fix leaked net.Conn (grpc#4633)

This happens when NewServerTransport() returns nil, nil. The rawConn is
closed when the transport is closed, which will never happen in this
case (since the returned transport is nil).

server.go

@@ -863,6 +863,7 @@ func (s *Server) handleRawConn(lisAddr string, rawConn net.Conn) {
 	// Finish handshaking (HTTP2)
 	st := s.newHTTP2Transport(conn, authInfo)
 	if st == nil {
+		conn.Close()
 		return
 	}