[RF] Set plot norm variables in RooAbsPdf::plotOn before using them
[RF] Set plot norm variables in RooAbsPdf::plotOn before using them

The first call to `frame->updateNormVars(*frame->getPlotVar())` should
be made before the first call to `frame->getNormVars()` in the
`RooAbsPdf::plotOn` function.

This fixes the Jira issue ROOT-5529.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root