ubifs: Fail commit if TNC is obviously inconsistent

A reference to LEB 0 or with length 0 in the TNC
is never correct and could be caused by a memory corruption.
Don't write such a bad index node to the MTD.
Instead fail the commit which will turn UBIFS into read-only mode.

This is less painful than having the bad reference on the MTD
from where UBFIS has no chance to recover.

Signed-off-by: Richard Weinberger <richard@nod.at>