unpack-trees: clear index when not propagating it

unpack-trees: clear index when not propagating it

When provided a pointer to a destination index, then `unpack_trees()`
will end up copying its `o->internal.result` index into the provided
pointer. In those cases it is thus not necessary to free the index, as
we have transferred ownership of it.

There are cases though where we do not end up transferring ownership of
the memory, but `clear_unpack_trees_porcelain()` will never discard the
index in that case and thus cause a memory leak. And right now it cannot
do so in the first place because we have no indicator of whether we did
or didn't transfer ownership of the index.

Adapt the code to zero out the index in case we transfer its ownership.
Like this, we can now unconditionally discard the index when being asked
to clear the `unpack_trees_options`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>