MDEV-35713 UBSAN: runtime error: load of value 1341112147, which is not a valid value for type 'enum enum_schema_tables' in optimize_for_get_all_tables and get_all_tables and on SELECT from I_S geometry_columns.

get_schema_table_idx() returns garbage if the SCHEMA table comes
from a plugin. Added specific value to reflect this.

sql/handler.h

@@ -1141,7 +1141,10 @@ enum enum_schema_tables
   SCH_TABLE_PRIVILEGES,
   SCH_TRIGGERS,
   SCH_USER_PRIVILEGES,
-  SCH_VIEWS
+  SCH_VIEWS,
+
+  SCH_N_SERVER_TABLES, /* How many SCHEMA tables in the server. */
+  SCH_PLUGIN_TABLE     /* Schema table defined in plugin. */
 };
 
 struct TABLE_SHARE;

sql/sql_show.cc

@@ -4292,6 +4292,12 @@ bool get_lookup_field_values(THD *thd, COND *cond, TABLE_LIST *tables,
 
 enum enum_schema_tables get_schema_table_idx(ST_SCHEMA_TABLE *schema_table)
 {
+  if (schema_table < schema_tables ||
+      schema_table > &schema_tables[SCH_N_SERVER_TABLES])
+  {
+    return SCH_PLUGIN_TABLE;
+  }
+
   return (enum enum_schema_tables) (schema_table - &schema_tables[0]);
 }