- revert to changes 31697 and redo 31693

- fix also case when fBinSumw2 is non zero (in GetEffectiveEntries) when reading an old file (< 5.22) with Sumw2 set 
This fixes stressHistogram


git-svn-id: http://root.cern.ch/svn/root/trunk@31737 27541ba8-7e3a-0410-8455-c3a389f83636