Fix a problem on Windows: fix getting a pointer to first element of a  std::vector when the vector is empty


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48815 27541ba8-7e3a-0410-8455-c3a389f83636
fix a compilation problem of the tutorial


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48816 27541ba8-7e3a-0410-8455-c3a389f83636