Correct length check in my_wc_mb_filename()

mysql-test/r/ctype_filename.result

@@ -11,3 +11,6 @@ create table com1 (a int);
 drop table com1;
 create table `clock$` (a int);
 drop table `clock$`;
+select convert(convert(',' using filename) using binary);
+convert(convert(',' using filename) using binary)
+@002c

mysql-test/t/ctype_filename.test

@@ -19,3 +19,6 @@ drop table com1;
 
 create table `clock$` (a int);
 drop table `clock$`;
+
+select convert(convert(',' using filename) using binary);
+

strings/ctype-utf8.c

@@ -4585,7 +4585,7 @@ my_wc_mb_filename(CHARSET_INFO *cs __attribute__((unused)),
   }
 
   /* Non letter */
-  if (s + 5 > e)
+  if (s + 4 > e)
     return MY_CS_TOOSMALL5;
 
   *s++= hex[(wc >> 12) & 15];