fix tests related to SQL comment length

tests are:
engines/funcs.jp_comment_column
engines/funcs.jp_comment_index
engines/funcs.jp_comment_table

sql/sql_string.h

@@ -79,7 +79,7 @@ class Well_formed_prefix: public Well_formed_prefix_status
   Well_formed_prefix(CHARSET_INFO *cs, const char *str, size_t length)
    :Well_formed_prefix_status(cs, str, str + length, length), m_str(str)
   { }
-  Well_formed_prefix(CHARSET_INFO *cs, LEX_STRING str, size_t nchars)
+  Well_formed_prefix(CHARSET_INFO *cs, LEX_CSTRING str, size_t nchars)
    :Well_formed_prefix_status(cs, str.str, str.str + str.length, nchars),
     m_str(str.str)
   { }

sql/sql_table.cc

@@ -4330,11 +4330,8 @@ bool validate_comment_length(THD *thd, LEX_CSTRING *comment, size_t max_len,
   if (comment->length == 0)
     DBUG_RETURN(false);
 
-  if (max_len > comment->length)
-    max_len= comment->length;
-
   size_t tmp_len=
-      Well_formed_prefix(system_charset_info, comment->str, max_len).length();
+      Well_formed_prefix(system_charset_info, *comment, max_len).length();
   if (tmp_len < comment->length)
   {
     if (thd->is_strict_mode())