NS3 core dump after a long runtime

The simulation crashes occasionally after a long runtime. It might be a counter overflow or something. 

```
assert failed. cond="packet->GetSize () >= header.GetLengthType ()", +53029.191157120s 0 file=../src/csma/model/csma-net-device.cc, line=759
terminate called without an active exception
Aborted (core dumped)
```