MDEV-30456 ALTER TABLE algorithm clause not replicated

Fixing the regression happening with galera.galera_toi_ddl_nonconflicting
The problem, surfaced by the test, was due to multi-statement used in the test.
The test issues two queries, as multi-statement:

--send ALTER TABLE t1 ADD COLUMN f3 INTEGER; INSERT INTO t1 (f1, f2) VALUES (DEFAULT, 123);

With this, the session's THD structure has in query_string member
the full multi-statement, and this was used as base query in
TOI replication rewriting.

The fix is to pass actual length of the ALTER statement part for
TOI replication.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>

sql/wsrep_mysqld.cc

@@ -1885,7 +1885,7 @@ static int wsrep_alter_table_query(THD *thd, uchar** buf, size_t* buf_len,
                                    Alter_info *alter_info)
 {
   String log_query;
-  log_query.append(thd->query());
+  log_query.append(thd->query(), thd->query_length());
 
   /*
      if user has specified the alter algorithm by session variable alter_algorithm