Fix implementation of RooSimultaneous::extendMode()

For returning the extend mode examine all the pdf in the list and
flag as CanExtend or MustExtend if just one of the pdf can be extended.

Fix also passing the extended flags when creating the sub-NLL in RooNLLVar::create().
If one of the sim component cannot be extended then switch off the extended flag for that component. Before it was passed for all components.

This fixes issue root-project#6894

roofit/roofitcore/src/RooNLLVar.cxx

@@ -225,9 +225,11 @@ RooNLLVar::RooNLLVar(const RooNLLVar& other, const char* name) :
 RooAbsTestStatistic* RooNLLVar::create(const char *name, const char *title, RooAbsReal& pdf, RooAbsData& adata,
             const RooArgSet& projDeps, const char* rangeName, const char* addCoefRangeName,
             Int_t nCPU, RooFit::MPSplit interleave, bool verbose, bool splitRange, bool binnedL) {
-  auto testStat = new RooNLLVar(name, title,
-      dynamic_cast<RooAbsPdf&>(pdf), adata,
-      projDeps, _extended, rangeName, addCoefRangeName, nCPU, interleave, verbose, splitRange, false, binnedL,
+  RooAbsPdf & thePdf = dynamic_cast<RooAbsPdf&>(pdf);
+  // check if pdf can be extended
+  bool extendedPdf = _extended & thePdf.canBeExtended();
+  auto testStat = new RooNLLVar(name, title, thePdf, adata,
+      projDeps, extendedPdf , rangeName, addCoefRangeName, nCPU, interleave, verbose, splitRange, false, binnedL,
       _integrateBinsPrecision);
   testStat->batchMode(_batchEvaluations);
   return testStat;