Item(id) panics in local implementation if the id is not a correct path

[acabarbaye]

in https://github.com/graymeta/stow/blob/973a61f346d598a566affb53c4698764a67df164/local/container.go#L132

info, err := os.Stat(path)
	if os.IsNotExist(err) {
		return nil, stow.ErrNotFound
	}
	if info.IsDir() {
		return nil, errors.New("unexpected directory")
	}

if the stat fails because the path is not valid and so if the error is differetn than not exist, info is nil and so the line info.IsDir panics with runtime error: invalid memory address or nil pointer dereference

The correct behaviour should be to return an error if stat fails