Fix add Rollup bug

[kangkaisen]

Fix 332

1 when add_column_statistics, we should use use _table->num_key_fields(), not column_statistic_fields.size(), as rollup table num_key_fields will less than base table column_statistic_fields.size()

2 when adding rollup table, the base table column_statistic_fields maybe empty.

[chaoyli]

I have review your code and core dump info carefully. It may be a little difficulty.

1. Adding rollup should not call LinkedSchemaChange Class. Because LinkedSchemaChange will not writer new data but create a hard link instead. It is not the semantics for adding rollup.
2. CHECK in add_column_statistics is valid aside from LinkedSchemaChange. A new method should to be figured out to address this problem.
   I will reproduce this bug in my machine, and help to address this.

[kangkaisen]

Add a special add_column_statistics method for linked_schema_change.

@chaoyli Hi, chaoyli. Please review. Thanks.

[morningman]

LGTM