MDEV-12258 InnoDB: Fix the bogus debug assertion introduced in MDEV-12219

After a partial rollback, an undo log segment that is empty
may carry a duplicate-looking top_undo_no.

Adjust the debug assertions and add a test.

mysql-test/suite/innodb/r/trigger_error.result

@@ -0,0 +1,28 @@
+CREATE TABLE t1 (i INT) ENGINE=InnoDB;
+CREATE TABLE t2 (i INT) ENGINE=InnoDB;
+CREATE OR REPLACE TRIGGER tr1
+AFTER UPDATE ON t2
+FOR EACH ROW
+INSERT INTO tlog (i) VALUES (1);
+INSERT IGNORE INTO t2 VALUES (1);
+CREATE TRIGGER IF NOT EXISTS tr2
+BEFORE INSERT ON t2
+FOR EACH ROW
+INSERT INTO tlog (i) VALUES (2);
+START TRANSACTION;
+INSERT INTO t1 VALUES (1);
+UPDATE t2 SET i = 3;
+ERROR 42S02: Table 'test.tlog' doesn't exist
+INSERT INTO t1 VALUES (2);
+INSERT INTO t2 VALUES (4);
+ERROR 42S02: Table 'test.tlog' doesn't exist
+UPDATE t1 SET i = 4 LIMIT 1;
+COMMIT;
+SELECT * FROM t1;
+i
+4
+2
+SELECT * FROM t2;
+i
+1
+DROP TABLE t1,t2;