Ok now I replaced track.seedRef() with dummySeedRef, defined as <pre>edm::RefToBase<TrajectorySeed> dummySeedRef;</pre>
I still get the same exception, but redoing the debugging gives a slightly different outcome:
<pre>
(gdb) up
#3  0x00002aaad903bb87 in edm::refcore::getProductPtr_<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> > > (ref=...)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/Common/interface/RefCoreGet.h:23
23            WrapperHolder product = ref.getProductPtr(typeid(T));
(gdb) up
#4  0x00002aaad9039659 in edm::getProduct<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> > > (ref=...)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/Common/interface/RefCoreGet.h:39
39          return refcore::getProductPtr_<T>(ref);
(gdb) up
#5  0x00002aaad9036026 in edm::refitem::GetPtrImpl<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra, edm::refhelper::FindUsingAdvance<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra>, unsigned int>::getPtr_ (product=..., key=@0x7fffffff584c: 0)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/Common/interface/RefItemGet.h:19
19              C const* prod = edm::template getProduct<C>(product);
(gdb) up
#6  0x00002aaad90327d1 in edm::getPtr<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra, edm::refhelper::FindUsingAdvance<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra>, unsigned int> (product=..., iKey=@0x7fffffff584c: 0)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/Common/interface/RefItemGet.h:41
41          T const* p=refitem::GetPtrImpl<C, T, F, KEY>::getPtr_(product, iKey);
(gdb) up
#7  0x00002aaad902f7a4 in edm::Ref<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra, edm::refhelper::FindUsingAdvance<std::vector<reco::TrackExtra, std::allocator<reco::TrackExtra> >, reco::TrackExtra> >::operator-> (this=0x2aab0b24f8b0)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/Common/interface/Ref.h:636
636         return getPtr<REF_FOR_VECTOR_ARGS>(product_.toRefCore(), key());
(gdb) up
#8  0x00002aaad902e51a in reco::Track::seedDirection (this=0x2aab0b24f700)
    at /afs/cern.ch/cms/sw/ReleaseCandidates/vol1/slc5_amd64_gcc481/cms/cmssw/CMSSW_7_1_X_2014-03-27-0200/src/DataFormats/TrackReco/interface/Track.h:104
104         PropagationDirection seedDirection() const {return extra_->seedDirection();}
(gdb) list
99          unsigned short found() const { return  numberOfValidHits(); }
100         /// Number of lost (=invalid) hits on track.
101         unsigned short lost() const {return  numberOfLostHits();  }
102
103         /// direction of how the hits were sorted in the original seed
104         PropagationDirection seedDirection() const {return extra_->seedDirection();}
105
106         /**  return the edm::reference to the trajectory seed in the original
107          *   seeds collection. If the collection has been dropped from the
108          *   Event, the reference may be invalid. Its validity should be tested,
(gdb) up
#9  0x00002aaad9051e1c in RecoTrackAccumulator::accumulateEvent<PileUpEventPrincipal> (this=0x2aaae644b940, e=..., iSetup=..., tracks=..., tracksExtras=..., hits=...)
    at /afs/cern.ch/work/g/giamman/fastsim/mixing/CMSSW_7_1_X_2014-03-27-0200/src/FastSimulation/Tracking/plugins/RecoTrackAccumulator.cc:95
95                                                             dummySeedRef ) );
(gdb) list
90                                                             track.innerPosition(), track.innerMomentum(), track.innerOk(),
91                                                             track.outerStateCovariance(), track.outerDetId(),
92                                                             track.innerStateCovariance(), track.innerDetId(),
93                                                             track.seedDirection(),
94                                                             //track.seedRef() ) );
95                                                             dummySeedRef ) );
96            NewTrackList_->back().setExtra( reco::TrackExtraRef( rTrackExtras, NewTrackExtraList_->size() - 1) );
97            // rechits:
98            for( trackingRecHit_iterator hit = track.recHitsBegin(); hit != track.recHitsEnd(); ++ hit ) {
99              NewHitList_->push_back( (*hits)[hit_counter] );
</pre>