Problem: GROUP_CONCAT on a multi-byte column can truncate
  in the middle of a multibyte character when applying
  group_concat_max_len limit. It produces an invalid
  multi-byte character in the result string.
  
The second, easier version - reusing old "warning_for_row" flag,
instead of introducing of "result_is_full" - which was
added in the previous commit.


mysql-test/r/func_gconcat.result:
  Adding test case
mysql-test/t/func_gconcat.test:
  Adding test case
sql/item_sum.cc:
  Adding well_formed_len() call not to cut
      in the middle of a multi-byte character.