disable SHOW I_S_table for built-in I_S tables

This fixes
MDEV-9538 Server crashes in check_show_access on SHOW STATISTICS
MDEV-9539 Server crashes in make_columns_old_format on SHOW GEOMETRY_COLUMNS
MDEV-9540 SHOW SPATIAL_REF_SYS and SHOW SYSTEM_VARIABLES return empty results with numerous warnings

mysql-test/r/show.result

+show statistics;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'statistics' at line 1
+show spatial_ref_sys
+--error ER_PARSE_ERROR
+show system_variables;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'spatial_ref_sys
+--error ER_PARSE_ERROR
+show system_variables' at line 2
+show geometry_columns;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'geometry_columns' at line 1
+show nonexistent;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'nonexistent' at line 1

mysql-test/t/show.test

+#
+# MDEV-9538 Server crashes in check_show_access on SHOW STATISTICS
+# MDEV-9539 Server crashes in make_columns_old_format on SHOW GEOMETRY_COLUMNS
+# MDEV-9540 SHOW SPATIAL_REF_SYS and SHOW SYSTEM_VARIABLES return empty results with numerous warnings
+#
+--error ER_PARSE_ERROR
+show statistics;
+--error ER_PARSE_ERROR
+show spatial_ref_sys
+--error ER_PARSE_ERROR
+show system_variables;
+--error ER_PARSE_ERROR
+show geometry_columns;
+--error ER_PARSE_ERROR
+show nonexistent;

sql/sql_show.cc

@@ -7374,12 +7374,14 @@ static my_bool find_schema_table_in_plugin(THD *thd, plugin_ref plugin,
     #   pointer to 'schema_tables' element
 */
 
-ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name)
+ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name,
+                                   bool *in_plugin)
 {
   schema_table_ref schema_table_a;
   ST_SCHEMA_TABLE *schema_table= schema_tables;
   DBUG_ENTER("find_schema_table");
 
+  *in_plugin= false;
   for (; schema_table->table_name; schema_table++)
   {
     if (!my_strcasecmp(system_charset_info,
@@ -7388,6 +7390,7 @@ ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name)
       DBUG_RETURN(schema_table);
   }
 
+  *in_plugin= true;
   schema_table_a.table_name= table_name;
   if (plugin_foreach(thd, find_schema_table_in_plugin,
                      MYSQL_INFORMATION_SCHEMA_PLUGIN, &schema_table_a))

sql/sql_show.h

@@ -117,7 +117,10 @@ bool schema_table_store_record(THD *thd, TABLE *table);
 void initialize_information_schema_acl();
 COND *make_cond_for_info_schema(THD *thd, COND *cond, TABLE_LIST *table);
 
-ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name);
+ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name, bool *in_plugin);
+static inline ST_SCHEMA_TABLE *find_schema_table(THD *thd, const char* table_name)
+{ bool unused; return find_schema_table(thd, table_name, &unused); }
+
 ST_SCHEMA_TABLE *get_schema_table(enum enum_schema_tables schema_table_idx);
 int make_schema_select(THD *thd,  SELECT_LEX *sel,
                        ST_SCHEMA_TABLE *schema_table);

sql/sql_yacc.yy

@@ -12865,9 +12865,10 @@ show_param:
         | IDENT_sys remember_tok_start wild_and_where
            {
              LEX *lex= Lex;
+             bool in_plugin;
              lex->sql_command= SQLCOM_SHOW_GENERIC;
-             ST_SCHEMA_TABLE *table= find_schema_table(thd, $1.str);
-             if (!table || !table->old_format)
+             ST_SCHEMA_TABLE *table= find_schema_table(thd, $1.str, &in_plugin);
+             if (!table || !table->old_format || !in_plugin)
              {
                my_parse_error(thd, ER_SYNTAX_ERROR, $2);
                MYSQL_YYABORT;