fix a bug in using option "W" for TGraphErrors
(see mail in RootTalk)
(bug was introduced in  http://root.cern.ch/viewvc/trunk/hist/hist/src/HFitImpl.cxx?view=log#rev41984 (releases >= 5.32)


git-svn-id: https://root.cern.ch/svn/root/trunk@48328 27541ba8-7e3a-0410-8455-c3a389f83636