Fix the reading of HepMcParticleLink_p1 in 21.3

Even in 21.0 HepMcParticleLinks pointing to GenEvents which aren't the first entry in the McEventCollection are using the event numbers rather than the position McEventCollection as the eventIndex. This is a manual sweep of !31000 (merged) from master.

FYI @tlari, @jassahsa, @vpascuzz, @golling

Generators/GeneratorObjectsTPCnv/src/HepMcParticleLinkCnv_p1.cxx

@@ -35,7 +35,7 @@ void HepMcParticleLinkCnv_p1::persToTrans( const HepMcParticleLink_p1* persObj,
     HepMcParticleLink::ExtendedBarCode( persObj->m_barcode,
                                         persObj->m_mcEvtIndex,
                                         EBC_MAINEVCOLL,
-                                        true);
+                                        persObj->m_mcEvtIndex==0);
   transObj->m_have_particle = false;
   transObj->m_particle = nullptr;