Bug: Processing of data packet when D2D-UE is not associated to any eNB/gNB

[stefanSchuhbaeck]

This can happen when handovers are active and a UE leaves a eNB but cannot attach to a new one. In this
case the UE is associated with a dummy master_Id of zero which does not have any data, leading to nullpointers
in the mac layer processing

See #174

[giovanninardini]

#174 has been merged. Thanks.