hepmc3 3.3.1 crash

Hello,

There is a crash reading one hepmc file, tested with versions 3.3.0 and 3.3.1. A small program to reproduce the problem:

ssh lxplus

cd /afs/cern.ch/user/a/akorneev/public/hepmc3

source env.sh
3.03.01

./hepmcread.exe /afs/cern.ch/user/a/akorneev/public/sherpa140-ee34.hepmc
Reading event #1 ...
...
Reading event #26 ...
Segmentation fault (core dumped)

There is no crash if the file is read by hepmc2-based program.