Fixes to JetQuarkLabel

Fixes to JetQuarkLabel:

• Fix

std::vector<ElementLink<xAOD::TruthParticleContainer> >::const_iterator endChild = (*pitr)->decayVtx()->outgoingParticleLinks().begin();

to

std::vector<ElementLink<xAOD::TruthParticleContainer> >::const_iterator endChild = (*pitr)->decayVtx()->outgoingParticleLinks().end();

@jchapman @pclark