MDEV-7050: MySQL#74603 - Assertion `comma_length > 0' failed in mysql_prepare_create_table

Incorrect array lenght for comma buffer.

mysql-test/r/create.result

@@ -2477,3 +2477,9 @@ t1	CREATE TABLE `t1` (
   `c` char(32) AS (convert(cast(n as char), char)) PERSISTENT
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+set @@session.collation_server=filename;
+create table t1(a enum('',''));
+Warnings:
+Note	1291	Column 'a' has duplicated value '' in ENUM
+drop table t1;
+set @@session.collation_server=default;

mysql-test/t/create.test

@@ -2053,3 +2053,11 @@ select * from t1;
 show create table t1;
 
 drop table t1;
+
+#
+# MDEV-7050: MySQL#74603 - Assertion `comma_length > 0' failed in mysql_prepare_create_table
+#
+set @@session.collation_server=filename;
+create table t1(a enum('',''));
+drop table t1;
+set @@session.collation_server=default;

sql/sql_table.cc

@@ -2986,9 +2986,10 @@ mysql_prepare_create_table(THD *thd, HA_CREATE_INFO *create_info,
                                                sql_field->interval_list);
         List_iterator<String> int_it(sql_field->interval_list);
         String conv, *tmp;
-        char comma_buf[4]; /* 4 bytes for utf32 */
+        char comma_buf[5]; /* 5 bytes for 'filename' charset */
+        DBUG_ASSERT(sizeof(comma_buf) >= cs->mbmaxlen);
         int comma_length= cs->cset->wc_mb(cs, ',', (uchar*) comma_buf,
-                                          (uchar*) comma_buf + 
+                                          (uchar*) comma_buf +
                                           sizeof(comma_buf));
         DBUG_ASSERT(comma_length > 0);
         for (uint i= 0; (tmp= int_it++); i++)