ddl: fix the case that fails to split a table

[zimulala]

If we fail to split a table, but the table is created, we will get an error as follows:

mysql> create database sbtest;
Query OK, 0 rows affected (0.09 sec)
mysql> use sbtest;
Database changed
mysql> CREATE TABLE `shtable` (
    ->   `id` int(11) NOT NULL AUTO_INCREMENT,
    ->    PRIMARY KEY (`id`)
    ->  );
ERROR 1050 (42S01): Table 'shtable' already exists

[zimulala]

/run-all-tests

[zimulala]

PTAL @disksing @coocood @shenli

[coocood]

LGTM

[shenli]

LGTM