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/t/trigger_error.test

@@ -0,0 +1,31 @@
+--source include/have_innodb.inc
+
+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);
+--error ER_NO_SUCH_TABLE
+UPDATE t2 SET i = 3;
+INSERT INTO t1 VALUES (2);
+--error ER_NO_SUCH_TABLE
+INSERT INTO t2 VALUES (4);
+UPDATE t1 SET i = 4 LIMIT 1;
+COMMIT;
+
+SELECT * FROM t1;
+SELECT * FROM t2;
+
+DROP TABLE t1,t2;