Added a test case for bug #11295.
item_buff.cc:
  Fixed bug #11295.
  This a correction for the patch of bug #11088 that takes into
  account a possible NULL values of the BLOB column.


sql/item_buff.cc:
  Fixed bug #11295.
  This a correction for the patch of bug #11088 that takes into
  account a possible NULL values of the BLOB column.
mysql-test/t/group_by.test:
  Added a test case for bug #11295.
mysql-test/r/group_by.result:
  Added a test case for bug #11295.