MDEV-24503 Assertion m_prebuilt->trx == thd_to_trx(m_user_thd) failed in ha_innobase::is_read_only

ha_innobase::check_if_supported_inplace_alter(): Invoke is_read_only()
only after calling update_thd(). In this way, the assertion failure
should be avoided even if the client was reconnected right before an
ALTER TABLE statement.

This bug was introduced by MDEV-23497.

storage/innobase/handler/handler0alter.cc

 /*****************************************************************************
 
 Copyright (c) 2005, 2019, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -1975,13 +1975,6 @@ ha_innobase::check_if_supported_inplace_alter(
 		DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
 	}
 
-	if (is_read_only()) {
-		ha_alter_info->unsupported_reason =
-			my_get_err_msg(ER_READ_ONLY_MODE);
-
-		DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
-	}
-
 	if (altered_table->s->fields > REC_MAX_N_USER_FIELDS) {
 		/* Deny the inplace ALTER TABLE. MySQL will try to
 		re-create the table and ha_innobase::create() will
@@ -1994,6 +1987,13 @@ ha_innobase::check_if_supported_inplace_alter(
 
 	update_thd();
 
+	if (is_read_only()) {
+		ha_alter_info->unsupported_reason =
+			my_get_err_msg(ER_READ_ONLY_MODE);
+
+		DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
+	}
+
 	if (ha_alter_info->handler_flags
 	    & ~(INNOBASE_INPLACE_IGNORE
 		| INNOBASE_ALTER_INSTANT