There is an assert failure when Window's child is a HashJoin operator
and while filling its buffer Window receives a NULL tuple. In this case
HashJoin will call ExecEagerFreeHashJoin() since it is done returning
any tuples. However, Window once it has returned all the tuples in its
input buffer will call ExecProcNode() on HashJoin. This casues an assert
failure in HashJoin that states that ExecHashJoin() should not be called
if HashJoin's hashtable has already been released.

This commit fixes the above issue by setting a flag in WindowState when
Window encounters a null tuple while filling its buffer. This flag then
guards any subsequent call to ExecProcNode() from fetchCurrentRow()