merge from trunk fix 49284  (https://sft.its.cern.ch/jira/browse/ROOT-4882 )

Fix THStack for 1D variable bin histograms


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49285 27541ba8-7e3a-0410-8455-c3a389f83636