MDEV-13564 follow-up: Harden an assertion

mlog_write_initial_log_record_low(): Do not allow the MLOG_TRUNCATE
record to be written.

storage/innobase/include/mtr0log.ic

 /*****************************************************************************
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation.
+Copyright (c) 2017, 2019, 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
@@ -187,7 +187,6 @@ mlog_write_initial_log_record_low(
 	      || type == MLOG_FILE_CREATE2
 	      || type == MLOG_FILE_RENAME2
 	      || type == MLOG_INDEX_LOAD
-	      || type == MLOG_TRUNCATE
 	      || type == MLOG_FILE_WRITE_CRYPT_DATA
 	      || mtr->is_named_space(space_id));