InnoDB leaks foreign key metadata on DDL operations

A memory leak was introduced in MDEV-6759 to address a ‘valgrind failure’ which looks like a double free when running ./mtr main.alter_table. Here is a simple test case that will trigger LeakSanitizer when building 10.1 with cmake -DWITH_ASAN=ON:

--source include/have_innodb.inc

CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=INNODB;

CREATE TABLE t2 (id INT PRIMARY KEY, t1_id INT, FOREIGN KEY (t1_id) REFERENCES t1(id)) ENGINE=INNODB;

DROP TABLE t2,t1;

With a fix in place, these memory leaks are gone, but there are some others. In 10.1.21, all of --suite=innodb,innodb_fts,innodb_zip,encryption completed fine. So did main.alter_table. Tests for --suite=main are still running.

I will test 10.0 too to see if a double free in main.alter_table can be observed there.