Bug #13943231: ALTER TABLE AFTER DISCARD MAY CRASH THE SERVER
Bug #13943231: ALTER TABLE AFTER DISCARD MAY CRASH THE SERVER

The following scenario crashes our mysql server:

1.  set global innodb_file_per_table=1;
2.  create table t1(c1 int) engine=innodb;
3.  alter table t1 discard tablespace;
4.  alter table t1 add unique index(c1);

Step 4 crashes the server.  This patch introduces a check on discarded
tablespace to avoid the crash.

rb://1041 approved by Marko Makela

natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,ollie314/server,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,slanterns/server,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,ollie314/server,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server