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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:
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Andrei Elkin <andrei.elkin@mariadb.com>