Change fSelectorUpdate from Bool_t to TSelector* to hold the value

of the selector being processed so that UpdateFormulaLeaves can update it
instead of blindly update the possibly
idle fSelector __and__ the possibly invalid (because of file reload)
fSelectorFromFile. (This resolves the crash in roottest/root/tree/selector)


git-svn-id: http://root.cern.ch/svn/root/trunk@17846 27541ba8-7e3a-0410-8455-c3a389f83636