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