Fix bug where pileup TrackingParticles are given the wrong initial vertex position

Fix to the old deprecated TrackingTruthProducer.  Bug shows up as a falsely improved dz resolution.  TrackingParticles are given their initial position from a SimVertex, but the array indexing doesn't take account of which crossing the SimVertex is in.  Consequently pileup TrackingParticles are given a position from a random SimVertex in the signal crossing.