Workaround for ROOT bugs

This is a workaround for the class edm::IndexIntoFile, which, in ROOT6 is affected by two ROOT6 bugs as described in:
https://sft.its.cern.ch/jira/browse/ROOT-5876?filter=-2
https://sft.its.cern.ch/jira/browse/ROOT-5880?filter=-2
Without this workaround, those ROOT6 bugs prevent any edm/ROOT file from being written by cmsRun.