repeating fix on 71x

Fix SimG4Core/Generators in 71x.
What's the status of this in 73X?
Is it still waiting for a forward port?

I see https://github.com/cms-sw/cmssw/blob/CMSSW_7_3_X/Validation/EventGenerator/src/HepMCValidationHelper.cc
(and the blame there suggests an interesting history of this assert coming in and going back and forth)

```
 assert(taudaughters.size()>0);
if ( taudaughters.size()==0 ) {
edm::LogError("HepMCValidationHelper") << "Tau with no daughters. This is a bug. Fix it";
continue;
}
```

My problem with this is a crash in a relval 1313 workflow (QCD)