MDEV-29008 Server crash or assertion `field' failed in spider_db_open_item_ident / group by handler

ha_spider::field_exchange() returns NULL and that results in a crash
or a assertion failure in spider_db_open_item_ident().

In the first place, there seems to be no need to call field_exchange()
for printing an identity (column name with alias). Thus, we simply
remove the call.

storage/spider/mysql-test/spider/bugfix/r/mdev_29008.result

+#
+# MDEV-29008 Server crash or assertion `field' failed in spider_db_open_item_ident / group by handler
+#
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+a INT,
+b INT
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+INSERT INTO tbl_a VALUES (1,2),(3,4);
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+a INT,
+b INT
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a", srv "s_2_1"';
+SELECT MIN(t2.a) AS f1, t1.b AS f2 FROM tbl_a AS t1 JOIN tbl_a AS t2 GROUP BY f2 ORDER BY f1, f2;
+f1	f2
+1	2
+1	4
+connection master_1;
+DROP DATABASE IF EXISTS auto_test_local;
+connection child2_1;
+DROP DATABASE IF EXISTS auto_test_remote;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_29008.cnf

+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_29008.test

+--echo #
+--echo # MDEV-29008 Server crash or assertion `field' failed in spider_db_open_item_ident / group by handler
+--echo #
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+eval CREATE TABLE tbl_a (
+    a INT,
+    b INT
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+INSERT INTO tbl_a VALUES (1,2),(3,4);
+
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+eval CREATE TABLE tbl_a (
+    a INT,
+    b INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a", srv "s_2_1"';
+
+SELECT MIN(t2.a) AS f1, t1.b AS f2 FROM tbl_a AS t1 JOIN tbl_a AS t2 GROUP BY f2 ORDER BY f1, f2;
+
+--connection master_1
+DROP DATABASE IF EXISTS auto_test_local;
+--connection child2_1
+DROP DATABASE IF EXISTS auto_test_remote;
+
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log

storage/spider/spd_db_conn.cc

@@ -9115,8 +9115,6 @@ int spider_db_open_item_ident(
         SPIDER_FIELD_HOLDER *field_holder = field_chain->field_holder;
         spider = field_holder->spider;
         share = spider->share;
-        field = spider->field_exchange(field);
-        DBUG_ASSERT(field);
         if ((error_num = share->dbton_share[dbton_id]->
           append_column_name_with_alias(str, field->field_index,
           field_holder->alias->ptr(), field_holder->alias->length())))