character set conversion of a string constant.


mysql-test/r/ctype_utf8.result:
  Adding test
mysql-test/t/ctype_utf8.test:
  Addign test
sql/item_strfunc.cc:
  Remove previous fix: it was not 100% correct.
  A non const_item() can return a constant String.
sql/sql_string.h:
  A new method to cut Alloced_length to str_length for
  string constants, to avoid reusing them as a buffer
  in things like CONCAT().