hotfix: turbo decoder should not fail if CRC is 0

The case of a CRC == 0 is legal.

After discussion with Raymond, it is also possible to have all
bits at 0 (and so a CRC==0) if there is no transmission and thus
not much energy.

So this hotfix may introduce new problems (false decoding).

A future work is to handle this case properly by not calling the
turbo decoder if there is not enough energy received.

The problem might manifest itself more in the UE part, especially
when it tries to decode MIB and/or SIB (if I understood correctly).