mysql-test/r/sp.result:
  New test case for BUG#3368.
mysql-test/t/sp.test:
  New test case for BUG#3368.
sql/item.h:
  Corrected const_item() for item_splocal. (Confusing semantics here, but this
  makes group_concat() behave as expected, instead of crashing, and does not
  appear to affect anything else.)