Prevent TFile::Close and TDirectory::Close from manipulating gDirectory.

This is necessary since there is no guarantee that the current directory
before the call to TFile::Close will still be a valid object after
the close (it might be deleted as a side-effect of the Close).


git-svn-id: http://root.cern.ch/svn/root/trunk@15808 27541ba8-7e3a-0410-8455-c3a389f83636