Keep the original PDG code for versions older than 8313

Author: pzhristov

Generators/src/GeneratorPythia8.cxx

@@ -654,12 +654,14 @@ Bool_t
     // first particle is system
     auto particle = event[iparticle];
     auto pdg = particle.id();
+#if PYTHIA_VERSION_INTEGER >= 8313
     // Select nuclei (A PDG id code for nuclei is on the form 100ZZZAAAI)
     if (std::abs(pdg) > 1000000000) {
       // Ignore isospin
       pdg /= 10;
       pdg *= 10;
     }
+#endif
     auto st = o2::mcgenstatus::MCGenStatusEncoding(particle.statusHepMC(), //
                                                    particle.status())      //
                 .fullEncoding;