Re-using an id after removing it is a bug in the caller but there are
two cases where we corrupt the free list without warning:
Removing an object twice:
id = pw_map_insert_new(object);
pw_map_remove(map, id);
pw_map_remove(map, id);
And inserting an element at an index previously removed:
id = pw_map_insert_new(object);
pw_map_remove(map, id);
pw_map_insert_at(map, id, new_object);
The latter is arguably valid code, or at least it'll look like it's
valid code.
For both cases, check if the id to remove/insert at is a free item and
handle that accordingly.
