Use address of member cast to volatile const char reference, instead of...

Use address of member cast to volatile const char reference, instead of address of member. Allows OffsetOf macro to work also for members of struct type where the struct has an overload of op&()


git-svn-id: http://root.cern.ch/svn/root/trunk@22843 27541ba8-7e3a-0410-8455-c3a389f83636