Problem:-
  create or replace table t1 (pk int auto_increment primary key invisible, i int);
  alter table t1 modify pk int invisible;
 This last alter makes a invisible column which is not null and does not
 have default value.

Analysis:-
 This is caused because our error check for NOT_NULL_FLAG and
 NO_DEFAULT_VALUE_FLAG flag misses this sql_field , but this is not the fault
 of error check :).Actually this field come via mysql_prepare_alter_table
 and it does not have NO_DEFAULT_VALUE_FLAG flag turned on. (If it was create
 table NO_DEFAULT_VALUE_FLAG would have turned on Column_definition::check)
 and this would have generated error.

Solution:-
 I have moved the error check to kind last of mysql_prepare_create_table
 because upto this point we have applied NO_DEFAULT_VALUE_FLAG to required
 column.