Fix in TH1::Add the case of average (bit kIsAverage set) when histograms have zero errors  see http://root.cern.ch/phpBB3//viewtopic.php?f=3&t=13299
Fix in TH1::Add the case of average (bit kIsAverage set) when histograms have zero errors 
see http://root.cern.ch/phpBB3//viewtopic.php?f=3&t=13299


git-svn-id: acec3fd5b7ea1eb9e79d6329d318e8118ee2e14f@40658 27541ba8-7e3a-0410-8455-c3a389f83636

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