MDEV-32060 Server aborts when table doesn't have referenced index

- Server aborts when table doesn't have referenced index.
This is caused by 5f09b53b (MDEV-31086).
While iterating the foreign key constraints, we fail to
consider that InnoDB doesn't have referenced index for
it when foreign key check is disabled.

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

@@ -81,4 +81,17 @@ DROP TABLE t2, t1;
 CREATE TABLE t (a VARCHAR(40), b INT, C INT) ENGINE=InnoDB;
 ALTER TABLE t MODIFY a VARCHAR(50), DROP b;
 DROP TABLE t;
+#
+#  MDEV-32060 Server aborts when table doesn't
+#		have referenced index
+#
+SET SESSION FOREIGN_KEY_CHECKS = OFF;
+CREATE TABLE t1 (a VARCHAR(16) KEY, FOREIGN KEY(a) REFERENCES t2(b)) ENGINE=InnoDB;
+CREATE TABLE t2 (b VARCHAR(8)) ENGINE=InnoDB;
+ERROR HY000: Can't create table `test`.`t2` (errno: 150 "Foreign key constraint is incorrectly formed")
+SET SESSION FOREIGN_KEY_CHECKS = ON;
+ALTER TABLE t2 MODIFY b VARCHAR(16), ADD KEY(b);
+ERROR 42S02: Table 'test.t2' doesn't exist
+DROP TABLE t2, t1;
+ERROR 42S02: Unknown table 'test.t2'
 # End of 10.4 tests

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

@@ -111,4 +111,19 @@ DROP TABLE t2, t1;
 CREATE TABLE t (a VARCHAR(40), b INT, C INT) ENGINE=InnoDB;
 ALTER TABLE t MODIFY a VARCHAR(50), DROP b;
 DROP TABLE t;
+
+--echo #
+--echo #  MDEV-32060 Server aborts when table doesn't
+--echo #		have referenced index
+--echo #
+SET SESSION FOREIGN_KEY_CHECKS = OFF;
+CREATE TABLE t1 (a VARCHAR(16) KEY, FOREIGN KEY(a) REFERENCES t2(b)) ENGINE=InnoDB;
+# Following SQL is allowed in 10.6
+--error ER_CANT_CREATE_TABLE
+CREATE TABLE t2 (b VARCHAR(8)) ENGINE=InnoDB;
+SET SESSION FOREIGN_KEY_CHECKS = ON;
+--error ER_NO_SUCH_TABLE
+ALTER TABLE t2 MODIFY b VARCHAR(16), ADD KEY(b);
+--error ER_BAD_TABLE_ERROR
+DROP TABLE t2, t1;
 --echo # End of 10.4 tests

storage/innobase/handler/handler0alter.cc

@@ -7712,6 +7712,8 @@ bool check_col_is_in_fk_indexes(
   {
     for (ulint i= 0; i < f->n_fields; i++)
     {
+      if (!f->referenced_index)
+        continue;
       if (f->referenced_index->fields[i].col == col)
       {
         my_error(ER_FK_COLUMN_CANNOT_CHANGE_CHILD, MYF(0),