bugfix: LDPC decoding in the gNB: clear d properly

The d array has to be cleared at the first usage, which may not necessarily be when new_rx is true. For example, in case of DTX detected for a first transmission, the next transmission won't have new_rx true, so d won't be cleared, containing data from a previous transmission. Or when a LDPC decoding is cancelled for a segment when new_rx is true, a decoding for a retransmission will not clear d because then new_rx will not be true. (Yes, the logic of cancellation will be changed in the future, but the changes will be orthogonal to the ones of this commit.)