More fixes for newer Pythia

In newer (>= 8.310) the `SubCollision` objects may not be stored with
the `HIInfo` object.  Thus, we cannot rely on those to be present when
calculating $N_{\mathrm{coll}}$, $N_{\mathrm{part}}$, and so on.

Instead, we must use the quantities stored directly in the `HIInfo`
object.

This PR fixes that issue by protecting on the Pythia version and falling
back to the information stored in `HIInfo`.

See also JIRA [issue](https://its.cern.ch/jira/browse/O2-4699).

Generators/src/GeneratorPythia8.cxx

@@ -752,14 +752,21 @@ void GeneratorPythia8::getNcoll(const Pythia8::Info& info, int& nColl)
 #else
   auto hiinfo = info.hiInfo;
 #endif
+  nColl = 0;
+  if (!hiinfo) {
+    LOG(warn) << "No heavy-ion information from Pythia";
+    return;
+  }
 
   // This is how the Pythia authors define Ncoll
   nColl = (hiinfo->nAbsProj() + hiinfo->nDiffProj() +
            hiinfo->nAbsTarg() + hiinfo->nDiffTarg() -
            hiinfo->nCollND() - hiinfo->nCollDD());
-  nColl = 0;
 
-  if (!hiinfo) {
+  if (not hiinfo->subCollisionsPtr()) {
+#if PYTHIA_VERSION_INTEGER < 8310
+    LOG(fatal) << "No sub-collision pointer from Pythia";
+#endif
     return;
   }
 
@@ -785,15 +792,21 @@ void GeneratorPythia8::getNpart(const Pythia8::Info& info, int& nPart)
 
   /** compute number of participants as the sum of all participants nucleons **/
 
-  // This is how the Pythia authors calculate Npart
 #if PYTHIA_VERSION_INTEGER < 8300
   auto hiinfo = info.hiinfo;
 #else
   auto hiinfo = info.hiInfo;
 #endif
-  if (hiinfo) {
-    nPart = (hiinfo->nAbsProj() + hiinfo->nDiffProj() +
-             hiinfo->nAbsTarg() + hiinfo->nDiffTarg());
+  nPart = 0;
+  if (not hiinfo) {
+    return;
+  }
+
+  // This is how the Pythia authors calculate Npart
+  nPart = (hiinfo->nAbsProj() + hiinfo->nDiffProj() +
+           hiinfo->nAbsTarg() + hiinfo->nDiffTarg());
+  if (not hiinfo->subCollisionsPtr()) {
+    return;
   }
 
   int nProtonProj, nNeutronProj, nProtonTarg, nNeutronTarg;
@@ -819,6 +832,15 @@ void GeneratorPythia8::getNpart(const Pythia8::Info& info, int& nProtonProj, int
     return;
   }
 
+  nProtonProj = hiinfo->nAbsProj() + hiinfo->nDiffProj();
+  nProtonTarg = hiinfo->nAbsTarg() + hiinfo->nDiffTarg();
+  if (not hiinfo->subCollisionsPtr()) {
+#if PYTHIA_VERSION_INTEGER < 8310
+    LOG(fatal) << "No sub-collision pointer from Pythia";
+#endif
+    return;
+  }
+
   // keep track of wounded nucleons
   std::vector<Pythia8::Nucleon*> projW;
   std::vector<Pythia8::Nucleon*> targW;