restore assert ValidationEventGenerator 4tau decays

restore assert ValidationEventGenerator 4tau decays; it'll re-introduced the check to asses the fix of #5119 

@inugent @bendavid : please confirm that indeed with #5119 already merged, the tau decay is now fixed.

And please, if that is the case, confirm in an email to cms-PPD-PdmV-val-admin@cern.ch that we need to re-do GEN-SIM for 72_pre6