BUG: potential memory leaks in HashPtrTable (#1787)

- using HashPtrTable::set() with the same key twice did not guarantee
  proper cleanup of memory since it simply used the underlying
  HashTable::set() without doing anything about the old memory. Now
  check for pre-existing storage and delete it when it does not
  correspond to the newly stored pointer.

  This problem is independent of potential memory slicing previously
  flagged (#1286) and only partially resolved.