MDEV-31869 Server aborts when table does drop column

- InnoDB aborts when table is dropping the column. This is
caused by 5f09b53b (MDEV-31086).
While iterating the altered table fields, we fail to consider
the dropped columns.

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

@@ -75,3 +75,10 @@ ALTER TABLE t1 MODIFY msg VARCHAR(200) CHARSET utf8mb4, ALGORITHM=COPY;
 ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY (msg) REFERENCES t1(msg), aLGORITHM=INPLACE;
 SET FOREIGN_KEY_CHECKS=1;
 DROP TABLE t2, t1;
+#
+# MDEV-31869 Server aborts when table does drop column
+#
+CREATE TABLE t (a VARCHAR(40), b INT, C INT) ENGINE=InnoDB;
+ALTER TABLE t MODIFY a VARCHAR(50), DROP b;
+DROP TABLE t;
+# End of 10.4 tests

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

@@ -104,3 +104,11 @@ ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY (msg) REFERENCES t1(msg), aLGORITHM=IN
 SET FOREIGN_KEY_CHECKS=1;
 
 DROP TABLE t2, t1;
+
+--echo #
+--echo # MDEV-31869 Server aborts when table does drop column
+--echo #
+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 # End of 10.4 tests

storage/innobase/handler/handler0alter.cc

@@ -8330,14 +8330,21 @@ ha_innobase::prepare_inplace_alter_table(
 	index columns are modified */
 	if (ha_alter_info->handler_flags
 			& ALTER_COLUMN_TYPE_CHANGE_BY_ENGINE) {
-		List_iterator<Create_field> it(
-			ha_alter_info->alter_info->create_list);
-		for (uint i = 0; i < table->s->fields; i++) {
+
+		for (uint i= 0; i < table->s->fields; i++) {
 			Field* field = table->field[i];
-			Create_field *f= it++;
-			if (!f->field || field->is_equal(*f))
-				continue;
+			for (const Create_field& new_field :
+				ha_alter_info->alter_info->create_list) {
+				if (new_field.field == field) {
+					if (!field->is_equal(new_field)) {
+						goto field_changed;
+					}
+					break;
+				}
+			}
 
+			continue;
+field_changed:
 			const char* col_name= field->field_name.str;
 			dict_col_t *col= dict_table_get_nth_col(
 				m_prebuilt->table, i);