Added a test case for bug #7769.
item_sum.h:
  Fixed bug #7769: a crash for queries with group_concat and
  having when the query table was empty.
  The bug was due an unsafe dereferencing.


sql/item_sum.h:
  Fixed bug #7769: a crash for queries with group_concat and
  having when the query table was empty.
  The bug was due an unsafe dereferencing.
mysql-test/t/func_gconcat.test:
  Added a test case for bug #7769.
mysql-test/r/func_gconcat.result:
  Added a test case for bug #7769.