Fix https://sft.its.cern.ch/jira/browse/ROOT-9854
Fix https://sft.its.cern.ch/jira/browse/ROOT-9854

In case the array of sums of TH1 was created before
painting the stack (for instance by calling GetMaximum() on the stack),
the automatic colouring of the stack was not working.

karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root