new_disk_conf could be leaked if the follow on checks fail,
so make sure to free it on error if it was not assigned yet.

Found with smatch.
Signed-off-by: NOleg Drokin <green@linuxhacker.ru>
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: NJens Axboe <axboe@fb.com>