HypoTestResult: return -1 instead of infinity (which crashes the plots) for Cls error when Clb=0
(CLs is = -1 in that case)

LikelihoodTestStats:  add option ReuseNLL on by default


git-svn-id: http://root.cern.ch/svn/root/trunk@40252 27541ba8-7e3a-0410-8455-c3a389f83636