resolving bugs in track_index assignment in TrigEgammaFastElectronReAlgo.cxx

As discussed in ATR-23500 the https://acode-browser1.usatlas.bnl.gov/lxr/source/athena/Trigger/TrigAlgorithms/TrigEgammaFastRec/src/TrigEgammaFastElectronReAlgo.cxx#0271 introduces a bug that interferes in line https://acode-browser1.usatlas.bnl.gov/lxr/source/athena/Trigger/TrigAlgorithms/TrigEgammaFastRec/src/TrigEgammaFastElectronReAlgo.cxx#0249

 ElementLink<xAOD::TrackParticleContainer> trackEL = ElementLink<xAOD::TrackParticleContainer> (*tracks, track_index);

While looping on tracks whenever a track fails the selection it brings the track_index to zero which results in selecting a failed track for the final electron. This MR resolves that issue. Adding @jodafons , @cjmeyer , @fernando