n TGraph::Fit and GraphFitChisquare, improvements by Eddy Offermann
to recompute the errors on parameters in case errors on the points
are not available (case of TGraph in particular).

Add a protection in TGraph::PaintFit in case the fit option was not
requested.


git-svn-id: http://root.cern.ch/svn/root/trunk@5018 27541ba8-7e3a-0410-8455-c3a389f83636