ubifs: fix possible dereference after free

'old_idx' could be dereferenced after free via 'rb_link_node' function
call.

Fixes: b5fda08e ("ubifs: Fix memleak when insert_old_idx() failed")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>