pack-bitmap: plug leak in find_objects()

pack-bitmap: plug leak in find_objects()

The `find_objects()` function creates an object_list for any tips of the
reachability query which do not have corresponding bitmaps.

The object_list is not used outside of `find_objects()`, but we never
free it with `object_list_free()`, resulting in a leak. Let's plug that
leak by calling `object_list_free()`, which results in t6113 becoming
leak-free.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>