MENT-2388 MDEV-37518 Check number of fields in mysql.servers before trying to read Options

Also added a test covering MDEV-36087

This is a followup of 6f1161a

Author: mariadb-YuchenPei

mysql-test/main/servers_upgrade.result

@@ -0,0 +1,21 @@
+#
+# MDEV-36087 MariDB 11.7.2 - /usr/sbin/mariadbd got signal 11
+#
+create server s foreign data wrapper mysql options (host "127.0.0.1", database "test", user "root", port 3306);
+show create server s;
+Server	Create Server
+s	CREATE SERVER `s` FOREIGN DATA WRAPPER mysql OPTIONS (host '127.0.0.1', database 'test', user 'root', port '3306');
+alter table mysql.servers drop column Options;
+show create server s;
+Server	Create Server
+s	CREATE SERVER `s` FOREIGN DATA WRAPPER mysql OPTIONS (host '127.0.0.1', database 'test', user 'root', port '3306');
+# restart
+show create server s;
+Server	Create Server
+s	CREATE SERVER `s` FOREIGN DATA WRAPPER mysql OPTIONS ();
+ALTER TABLE mysql.servers
+ADD Options JSON NOT NULL DEFAULT '{}' CHECK(JSON_VALID(Options));
+show create server s;
+Server	Create Server
+s	CREATE SERVER `s` FOREIGN DATA WRAPPER mysql OPTIONS ();
+drop server s;

mysql-test/main/servers_upgrade.test

@@ -0,0 +1,17 @@
+--echo #
+--echo # MDEV-36087 MariDB 11.7.2 - /usr/sbin/mariadbd got signal 11
+--echo #
+
+# This test simulates the scenario in MDEV-36087 by dropping the
+# column added in MDEV-34716
+
+create server s foreign data wrapper mysql options (host "127.0.0.1", database "test", user "root", port 3306);
+show create server s;
+alter table mysql.servers drop column Options;
+show create server s;
+--source include/restart_mysqld.inc
+show create server s;
+ALTER TABLE mysql.servers
+  ADD Options JSON NOT NULL DEFAULT '{}' CHECK(JSON_VALID(Options));
+show create server s;
+drop server s;

sql/sql_servers.cc

@@ -473,7 +473,7 @@ get_server_from_table_to_cache(TABLE *table)
   server->scheme= ptr ? ptr : blank;
   ptr= get_field(&mem, table->field[8]);
   server->owner= ptr ? ptr : blank;
-  ptr= table->field[9] ? get_field(&mem, table->field[9]) : NULL;
+  ptr= table->s->fields > 9 ? get_field(&mem, table->field[9]) : NULL;
   server->option_list= NULL;
   if (ptr && parse_server_options_json(server, ptr))
     DBUG_RETURN(TRUE);