BUG#19192 - CHECK TABLE EXTENDED / REPAIR TABLE show no errors.

            ALTER TABLE crashes
Executing fast alter table (one that doesn't need to copy data)
on tables created by mysql versions prior to 4.0.25 could result
in posterior server crash when accessing these tables.

There was a bug prior to mysql-4.0.25. Number of null fields was
calculated incorrectly. As a result frm and data files gets out of
sync after fast alter table. There is no way to determine by which
mysql version (in 4.0 and 4.1 branches) table was created, thus we
disable fast alter table for all tables created by mysql versions
prior to 5.0 branch.
See BUG#6236.

sql/sql_table.cc

@@ -3614,12 +3614,21 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
     their layout. See Field_string::type() for details.
     Thus, if the table is too old we may have to rebuild the data to
     update the layout.
+
+    There was a bug prior to mysql-4.0.25. Number of null fields was
+    calculated incorrectly. As a result frm and data files gets out of
+    sync after fast alter table. There is no way to determine by which
+    mysql version (in 4.0 and 4.1 branches) table was created, thus we
+    disable fast alter table for all tables created by mysql versions
+    prior to 5.0 branch.
+    See BUG#6236.
   */
   need_copy_table= (alter_info->flags &
                     ~(ALTER_CHANGE_COLUMN_DEFAULT|ALTER_OPTIONS) ||
                     (create_info->used_fields &
                      ~(HA_CREATE_USED_COMMENT|HA_CREATE_USED_PASSWORD)) ||
                     table->s->tmp_table ||
+                    !table->s->mysql_version ||
                     (table->s->frm_version < FRM_VER_TRUE_VARCHAR && varchar));
   create_info->frm_only= !need_copy_table;