Fix memory leak on cpp object api (#5761)

Fix memory leak on cpp object api (#5761)

Previously UnPack would allocate data with new and assign it to a
raw pointer. This behavior makes it possible for the pointer to be
leaked in case of OOM. This commit defaults to use the user specified
pointer (which needs to implement a move constructor, a .get() and a
 .release() operators), thus preventing these leaks.