Protect TCanvas::Update in case fCanvasImp is not yet set, eg when calling Update

on a canvas just read from a file and not yet drawn.
This fixes the report http://root.cern.ch/root/roottalk/roottalk08/0271.html


git-svn-id: http://root.cern.ch/svn/root/trunk@22884 27541ba8-7e3a-0410-8455-c3a389f83636