hadd crashes when slow merging file with multiple array with same index

Seen with CMS NanoAOD files: https://root-forum.cern.ch/t/merge-nanoaod-root-files/39707/13

When the files have several array branches using the same "leaf count" branch and the maximum array size increase from file to file, only the memory allocated for the "first" array is extended. This results in memory error for the other arrays.