xml: fix problem with comments at the end of file
xml: fix problem with comments at the end of file

Reported here: https://root-forum.cern.ch/t/27671
root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,karies/root