ha_innobase::add_index(): Only check for duplicate indexes

when the data dictionary is locked.
This fixes a UNIV_DEBUG assertion failure in innodb-index.test.

storage/innodb_plugin/handler/handler0alter.cc

@@ -920,9 +920,8 @@ ha_innobase::add_index(
 		trx_commit_for_mysql(prebuilt->trx);
 	}
 
-	ut_d(dict_table_check_for_dup_indexes(innodb_table, FALSE));
-
 	if (dict_locked) {
+		ut_d(dict_table_check_for_dup_indexes(innodb_table, FALSE));
 		row_mysql_unlock_data_dictionary(trx);
 	}