Add protection against sqrt(negative value) due to numerical rounding ef...

...fects

Fixes pre9 relval crashes observed here: https://hypernews.cern.ch/HyperNews/CMS/get/relval/2765.html

```
between 1 and 6 percent of the HLTD jobs failed due to this error

An exception of category 'FatalRootError' occurred while
   [0] Processing run: 208307 lumi: 263 event: 412070218
   [1] Running path 'HLT_Ele30_CaloIdVT_TrkIdT_PFNoPUJet100_PFNoPUJet25_v9'
   [2] Calling event method for module HLTPFJetCollectionsForElePlusJets/'hltEle30CaloIdVTTrkIdTCleanAK5PFNoPUJet'
   Additional Info:
      [a] Fatal Root Error: @SUB=TVector3::PseudoRapidity
transvers momentum = 0! return +/- 10e10

```
 
Essentially, due to numerical rounding, a sqrt of s slightly negative number is taken,
which produces a "nan" which ends up as the jetarea and has knock on effects 
when calculating jet energy corrections updating the jet kinematics and thus the
jet eta.