Fixed bug #12885.
  Forced inheritence of the maybe_null flag for the expressions
  containing GROUP BY attributes in selects with ROLLUP.
olap.test, olap.result:
  Added test case for bug #12885.


mysql-test/r/olap.result:
  Added test case for bug #12885.
mysql-test/t/olap.test:
  Added test case for bug #12885.
sql/sql_select.cc:
  Fixed bug #12885.
  Forced inheritence of the maybe_null flag for the expressions
  containing GROUP BY attributes in selects with ROLLUP.