Type confusion

A type confusion issue was discovered in CCN-lite 2, leading to a memory access violation and a failure of the nonce feature (which, for example, helped with loop prevention). ccnl_fwd_handleInterest assumes that the union member s is of type ccnl_pktdetail_ndntlv_s. However, if the type is in fact struct ccnl_pktdetail_ccntlv_s or struct ccnl_pktdetail_iottlv_s, the memory at that point is either uninitialised or points to data that is not a nonce, which renders the code using the local variable nonce pointless. A later nonce check is insufficient.