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

storage/innodb_plugin/handler/handler0alter.cc

@@ -665,6 +665,10 @@ ha_innobase::add_index(
 		DBUG_RETURN(HA_ERR_NO_SUCH_TABLE);
 	}
 
+	if (innodb_table->tablespace_discarded) {
+		DBUG_RETURN(-1);
+	}
+
 	/* Check that index keys are sensible */
 	error = innobase_check_index_keys(key_info, num_of_keys, innodb_table);