MDEV-22651: Bogus assertion in dict_table_t::init_instant()

In commit 0e5a4ac2 (MDEV-15562)
we introduced a bogus debug assertion, demanding that dict_col_t::len
never exceed some maximum value. The intention was to match what
dict_index_add_col() is doing. But, the assignment field->fixed_len = 0
applies to dict_field_t::fixed_len, not dict_col_t::len!

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

@@ -399,4 +399,12 @@ INSERT INTO t1 SET a=1, b=NULL;
 ALTER TABLE t1 MODIFY COLUMN b INT FIRST;
 ALTER TABLE t1 ADD UNIQUE INDEX (va);
 DROP TABLE t1;
+#
+# MDEV-22651 Assertion dict_col_get_fixed_size...
+# in dict_table_t::init_instant()
+#
+CREATE TABLE t (i INT PRIMARY KEY) ENGINE=InnoDB;
+INSERT INTO t SET i=1;
+ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
+DROP TABLE t;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

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

@@ -419,4 +419,14 @@ ALTER TABLE t1 MODIFY COLUMN b INT FIRST;
 ALTER TABLE t1 ADD UNIQUE INDEX (va);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-22651 Assertion dict_col_get_fixed_size...
+--echo # in dict_table_t::init_instant()
+--echo #
+--source include/have_innodb.inc
+CREATE TABLE t (i INT PRIMARY KEY) ENGINE=InnoDB;
+INSERT INTO t SET i=1;
+ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
+DROP TABLE t;
+
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

storage/innobase/handler/handler0alter.cc

@@ -194,8 +194,6 @@ inline void dict_table_t::init_instant(const dict_table_t& table)
 	ut_d(unsigned n_nullable = 0);
 	for (unsigned i = u; i < index.n_fields; i++) {
 		auto& f = index.fields[i];
-		DBUG_ASSERT(dict_col_get_fixed_size(f.col, not_redundant())
-			    <= DICT_MAX_FIXED_COL_LEN);
 		ut_d(n_nullable += f.col->is_nullable());
 
 		if (!f.col->is_dropped()) {