The problem was that some functions (namely IN() starting with 4.1, and
CHAR() starting with 5.0) were returning NULL in certain conditions,
while they didn't set their maybe_null flag.  Because of that there could
be some problems with 'IS NULL' check, and statements that depend on the
function value domain, like CREATE TABLE t1 SELECT 1 IN (2, NULL);.

The fix is to set maybe_null correctly.


mysql-test/r/func_in.result:
  Add result for bug#17047: CHAR() and IN() can return NULL without
  signaling NULL result.
mysql-test/t/func_in.test:
  Add test case for bug#17047: CHAR() and IN() can return NULL without
  signaling NULL result.
sql/item_cmpfunc.cc:
  Remove assignment to maybe_null, as it was already set in fix_fields()
  based on all arguments, not only on the first.