The problem is that argument buffer can be used as result buffer
and it leads to argument value change.
The fix is to use 'old buffer' as result buffer only
if first argument is not constant item.


mysql-test/r/func_str.result:
  test result
mysql-test/t/func_str.test:
  test case
sql/item_strfunc.cc:
  The problem is that argument buffer can be used as result buffer
  and it leads to argument value change.
  The fix is to use 'old buffer' as result buffer only
  if first argument is not constant item.