MDEV-26919: binlog.binlog_truncate_active_log fails in bb with valgrind,...

MDEV-26919: binlog.binlog_truncate_active_log fails in bb with valgrind, Conditional jump or move depends on uninitialised value

Problem:
========
When writing an XA based event to the binary log, an assert was
always referencing thd->lex->xa_opt. This variable, however, is
only set when using XA START, XA END, and XA COMMIT. When an
XA PREPARE statement is being processed, it is not guaranteed that
the xa_opt variable will be set (e.g. if existing within a stored
procedure). This caused valgrind to complain about accessing an
uninitialized variable.

Solution:
========
Before referencing xa_opt, ensure the context is valid such that
it is set.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>

sql/log_event_server.cc

@@ -3281,7 +3281,8 @@ Gtid_log_event::Gtid_log_event(THD *thd_arg, uint64 seq_no_arg,
       (thd->lex->sql_command == SQLCOM_XA_PREPARE ||
        xid_state.get_state_code() == XA_PREPARED))
   {
-    DBUG_ASSERT(thd->lex->xa_opt != XA_ONE_PHASE);
+    DBUG_ASSERT(!(thd->lex->sql_command == SQLCOM_XA_COMMIT &&
+                  thd->lex->xa_opt == XA_ONE_PHASE));
 
     flags2|= thd->lex->sql_command == SQLCOM_XA_PREPARE ?
       FL_PREPARED_XA : FL_COMPLETED_XA;