MDEV-29314 Assertion `n_fields > n_cols' failed in dict_index_t::init_change_cols

- Newly created InnoDB fulltext index does have only one column
and doesn't associate with primary key fields during DDL.
init_change_cols() has strict assertion that number of fields
should be greater than number of collation change columns.

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

@@ -95,3 +95,17 @@ ALTER TABLE t1 MODIFY msg_2 VARCHAR(100) CHARACTER SET utf8
 COLLATE utf8_unicode_ci, ALGORITHM=inplace;
 ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
 DROP TABLE t1;
+#
+# MDEV-29314 Assertion `n_fields > n_cols' failed
+#	in dict_index_t::init_change_cols
+#
+CREATE TABLE t (a VARCHAR(16) COLLATE utf8_bin,
+FULLTEXT (a)) ENGINE=InnoDB COLLATE utf8_unicode_520_ci;
+ALTER TABLE t MODIFY COLUMN a VARCHAR(512);
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `a` varchar(512) COLLATE utf8mb3_unicode_520_ci DEFAULT NULL,
+  FULLTEXT KEY `a` (`a`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_unicode_520_ci
+DROP TABLE t;

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

@@ -122,4 +122,14 @@ ALTER TABLE t1 MODIFY msg_2 VARCHAR(100) CHARACTER SET utf8
                 COLLATE utf8_unicode_ci, ALGORITHM=inplace;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-29314 Assertion `n_fields > n_cols' failed
+--echo #	in dict_index_t::init_change_cols
+--echo #
+CREATE TABLE t (a VARCHAR(16) COLLATE utf8_bin,
+		FULLTEXT (a)) ENGINE=InnoDB COLLATE utf8_unicode_520_ci;
+ALTER TABLE t MODIFY COLUMN a VARCHAR(512);
+SHOW CREATE TABLE t;
+DROP TABLE t;
+
 --source include/wait_until_count_sessions.inc

storage/innobase/include/dict0mem.h

@@ -1345,7 +1345,7 @@ struct dict_index_t {
   @param  n_cols   number of columns whose collation is changing */
   void init_change_cols(unsigned n_cols)
   {
-    ut_ad(n_fields > n_cols);
+    ut_ad(n_fields > n_cols || type & DICT_FTS);
     change_col_info= static_cast<col_info*>
       (mem_heap_zalloc(heap, sizeof(col_info)));
     change_col_info->n_cols= n_cols;