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>

pack-bitmap.c

@@ -1280,6 +1280,8 @@ static struct bitmap *find_objects(struct bitmap_index *bitmap_git,
 		base = fill_in_bitmap(bitmap_git, revs, base, seen);
 	}
 
+	object_list_free(&not_mapped);
+
 	return base;
 }