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

@@ -2234,10 +2234,10 @@ static int wsrep_alter_event_query(THD *thd, uchar** buf, size_t* buf_len)
 }
 
 static int wsrep_alter_table_query(THD *thd, uchar** buf, size_t* buf_len,
-                                   Alter_info *alter_info)
+                                   const 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
@@ -2247,8 +2247,8 @@ static int wsrep_alter_table_query(THD *thd, uchar** buf, size_t* buf_len,
   if (thd->variables.alter_algorithm  != Alter_info::ALTER_TABLE_ALGORITHM_DEFAULT &&
       alter_info->requested_algorithm == Alter_info::ALTER_TABLE_ALGORITHM_NONE)
   {
-    if (log_query.append(" ,") ||
-        log_query.append(alter_info->algorithm_clause(thd)))
+    if (log_query.append(MYF(" ,")) ||
+        log_query.append(MYF(alter_info->algorithm_clause(thd))))
     {
       WSREP_WARN("alter table string failed: schema: %s, query: %s",
                  thd->get_db(), thd->query());
@@ -2630,7 +2630,8 @@ static int wsrep_create_sp(THD *thd, uchar** buf, size_t* buf_len)
   return wsrep_to_buf_helper(thd, log_query.ptr(), log_query.length(), buf, buf_len);
 }
 
-static int wsrep_TOI_event_buf(THD* thd, uchar** buf, size_t* buf_len, Alter_info *alter_info)
+static int wsrep_TOI_event_buf(THD* thd, uchar** buf, size_t* buf_len,
+                               const Alter_info *alter_info)
 {
   int err;
   switch (thd->lex->sql_command)