server: fix leaked net.Conn (grpc#4644)

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).

This is a backport of grpc#4633

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
 	}