fix tests related to SQL comment length

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

Author: kevgs

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())