#9 suggest improved treatment of back pointer in doubly linked

[Chris Henrich]

In bc-containers-lists-double.adb, in the procedure Clear(R: in out double_Node_Ref) the code that reads
if Curr /= null then
Curr.Previous := null;
end if;
should be changed to
if Curr /= null and then Curr.Previus = Ptr then
Curr.Previous := null;
end if;
With this change, the removable of unreachable items from memory will have no surprising side effects on other items.