When using GROUP_CONCAT with ORDER BY, a tree is used for the sorting, as 
opposed to normal nested loops join used when there is no ORDER BY. 

The tree traversal that generates the result counts the lines that have been 
cut down. (as they get cut down to the field's max_size)
But the check of that count was before the tree traversal, so no 
warning was generated if the output is truncated.

Fixed by moving the check to after the tree traversal.


mysql-test/r/func_gconcat.result:
  bug#28273: correct result
mysql-test/t/func_gconcat.test:
  bug#28273: test case
sql/item_sum.cc:
  bug#28273: the fix
  
  Moved the code that outputs a warning to after temporary table (tree) is traversed.