basic simulator: bugfix: do not put random data in DCI when not used

With 50 RBs and 100 RBs the UE crashes in the basic simulator
because of false detection of DCIs.

Putting nothing in the unused REs in the DCI instead of random
bits solves this issue and let the basic simulator run with 50 RBs
and 100 RBs.

Note that in the real UE the problem needs to be solved because
the channel may lead to false DCI detection and the consequent
crashes have to be solved. This commit sort of "hides" the issue
to have the basic simulator functional.

Note also that putting nothing in the unused REs in the DCI should
be the normal general case. But it has not been tested with anything
but the basic simulator so the previous code is left.