MDEV-22520 Assertion gathered_length == thd->lex->comment.length failed in binlog_defragment

The assert was caused by early cleanup of a user variable participant
in BINLOG @var,@var where it plays twice. That was unexpected by the base
code to clear its value prematurely.

Fixed with relocating the user var destruction after operations with
its value is over.

mysql-test/suite/binlog/t/binlog_mysqlbinlog_row_frag.test

@@ -40,6 +40,11 @@ SET @binlog_fragment_0='012345';
 --error ER_WRONG_TYPE_FOR_VAR
 BINLOG @binlog_fragment_0, @binlog_fragment_not_exist;
 
+# MDEV-22520
+SET @a= '42';
+--error ER_SYNTAX_ERROR
+BINLOG @a, @a;
+
 --echo # Cleanup
 --remove_file $MYSQLTEST_VARDIR/tmp/mysqlbinlog.sql
 DROP TABLE t;