MDEV-19092 Server crash when renaming the column when

			FOREIGN_KEY_CHECKS is disabled

- dict_foreign_find_index() can return NULL if InnoDB already dropped
the foreign index when FOREIGN_KEY_CHECKS is disabled.

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

@@ -113,3 +113,10 @@ CREATE TABLE t2 (f INT, KEY(f)) ENGINE=InnoDB;
 ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t2 (f);
 ALTER IGNORE TABLE t1 ADD FOREIGN KEY (f3) REFERENCES t1 (f1);
 DROP TABLE t1, t2;
+CREATE TABLE t1 (a INT, b INT, KEY idx(a)) ENGINE=InnoDB;
+SET FOREIGN_KEY_CHECKS= OFF;
+ALTER TABLE t1 ADD FOREIGN KEY (a) REFERENCES tx(x);
+ALTER TABLE t1 DROP KEY idx;
+ALTER TABLE t1 CHANGE a c INT;
+DROP TABLE t1;
+SET FOREIGN_KEY_CHECKS=1;

mysql-test/suite/innodb/t/foreign_key.test

@@ -134,3 +134,14 @@ CREATE TABLE t2 (f INT, KEY(f)) ENGINE=InnoDB;
 ALTER TABLE t1 ADD FOREIGN KEY (f2) REFERENCES t2 (f);
 ALTER IGNORE TABLE t1 ADD FOREIGN KEY (f3) REFERENCES t1 (f1);
 DROP TABLE t1, t2;
+
+# MDEV-19092 Server crash when renaming the column when
+# FOREIGN_KEY_CHECKS is disabled
+CREATE TABLE t1 (a INT, b INT, KEY idx(a)) ENGINE=InnoDB;
+SET FOREIGN_KEY_CHECKS= OFF;
+ALTER TABLE t1 ADD FOREIGN KEY (a) REFERENCES tx(x);
+ALTER TABLE t1 DROP KEY idx;
+ALTER TABLE t1 CHANGE a c INT;
+# Cleanup
+DROP TABLE t1;
+SET FOREIGN_KEY_CHECKS=1;

storage/innobase/dict/dict0mem.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -420,9 +420,10 @@ dict_mem_table_col_rename_low(
 				foreign->foreign_col_names,
 				foreign->n_fields, NULL, true, false,
 				NULL, NULL, NULL);
-			/* There must be an equivalent index in this case. */
-			ut_ad(new_index != NULL);
 
+			/* New index can be null if InnoDB already dropped
+			the foreign index when FOREIGN_KEY_CHECKS is
+			disabled */
 			foreign->foreign_index = new_index;
 
 		} else {

storage/xtradb/dict/dict0mem.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -421,9 +421,10 @@ dict_mem_table_col_rename_low(
 				foreign->foreign_col_names,
 				foreign->n_fields, NULL, true, false,
 				NULL, NULL, NULL);
-			/* There must be an equivalent index in this case. */
-			ut_ad(new_index != NULL);
 
+			/* New index can be null if XtraDB already dropped
+			the foreign index when FOREIGN_KEY_CHECKS is
+			disabled */
 			foreign->foreign_index = new_index;
 
 		} else {