Avoid dropping information when reading a long written on a 64 bits platforms

and being read into a long long on a 32 bits platform (previously the higher
bits were lost due to passing through a 32 bits temporary long).


git-svn-id: http://root.cern.ch/svn/root/trunk@37243 27541ba8-7e3a-0410-8455-c3a389f83636