The problem is that cond->fix_fields(thd, 0) breaks
condition(cuts off 'having'). The reason of that is
that NULL valued Item pointer is present in the
middle of Item list and it breaks the Item processing
loop.


mysql-test/r/having.result:
  test case
mysql-test/t/having.test:
  test case
sql/item_cmpfunc.h:
  added ASSERT to make sure that we do not add NULL valued Item pointer
sql/sql_select.cc:
  skip adding an item to condition if Item pointer is NULL.
  skip adding a list to condition if this list is empty.