Fix bug in MergeTruthJetsTool where only the first event from each bunch-crossing was checked.

This one is needed for the Presampled RDO production for the Run-2 Reprocessing.

Thanks to @dandoy for spotting this bug which had been lurking for 3 years!

FYI @tlari, @mduehrss, @tadej, @pberta, @rmazini, @emoyse, @elmsheus