Add protection against sqrt(negative value) due to numerical rounding ef... #1752
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...fects
Fixes pre9 relval crashes observed here: https://hypernews.cern.ch/HyperNews/CMS/get/relval/2765.html
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.