InnoDB assertion with result != FTS_INVALID on DELETE IGNORE with FK cascades on tables with FULLTEXT indexes

Problem:
========
DELETE IGNORE on a parent table crashes the server when an ON DELETE CASCADE
foreign key leads to a child table with a FULLTEXT index. The crash happens if the cascaded
delete of the child row fails with an error that IGNORE suppresses.
The failure here is a RESTRICT foreign key from a grandchild table.
It then happens again on a later parent row that cascades into the same child row

Test case:
=========

--source include/have_innodb.inc

CREATE TABLE t1 (id INT PRIMARY KEY, a VARCHAR(10), KEY(a)) ENGINE=InnoDB;

CREATE TABLE t2 (id INT PRIMARY KEY, a VARCHAR(10), b VARCHAR(10),

  KEY(a), KEY(b), FULLTEXT(a),

  FOREIGN KEY (a) REFERENCES t1(a) ON DELETE CASCADE) ENGINE=InnoDB;

CREATE TABLE t3 (id INT PRIMARY KEY, b VARCHAR(10), KEY(b),

  FOREIGN KEY (b) REFERENCES t2(b) ON DELETE RESTRICT) ENGINE=InnoDB;

INSERT INTO t1 VALUES (1,'xxx'),(2,'xxx');

INSERT INTO t2 VALUES (1,'xxx','k');

INSERT INTO t3 VALUES (1,'k');

DELETE IGNORE FROM t1;

SELECT * FROM t1;

SELECT * FROM t2 WHERE MATCH(a) AGAINST('xxx');

DROP TABLE t3, t2, t1;

Analysis:
==========
1) When a cascade is processed, row_ins_foreign_check_on_constraint()
calls fts_trx_add_op(..., FTS_DELETE) for the child row if the foreign
key affects a fulltext column. This happens before the outcome of the
cascaded delete is known.
2) The cascaded delete then fails because of the RESTRICT constraint
3) row_update_for_mysql_using_upd_graph() passes the error to row_mysql_handle_errors(). That function rolls back the data changes
to the internal savepoint with trx->rollback(savept). It does not touch
the FTS bookkeeping in trx->fts_trx.
4) The FTS_DELETE recorded for the child row stays in trx->fts_trx->last_stmt,
even though the row delete was rolled back.
5) The next parent row cascades into the same child row and records
FTS_DELETE again. The FTS state table in fts_trx_row_get_new_state() has no
valid transition from D on event D. It returns FTS_INVALID, and the assertion fails.

This is to merge the mysql commit@ac91b1506af34e19b1cb936691310b9268bc0dd8