PR 5990

	PR 5990
	* descriptors.h (Open_descriptor): Add is_on_stack field.
	* descriptors.cc (Descriptors::open): If the descriptor is on the
	top of the stack, remove it.  Initialize is_on_stack field.
	(Descriptors::release): Only add pod to stack if it is not on the
	stack already.
	(Descriptors::close_some_descriptor): Clear stack_next and
	is_on_stack fields.