adopt PFitter to ROOT v6-38-00 and make sure it is backwards compatible.

src/classes/PFitter.cpp

@@ -2683,7 +2683,7 @@ Bool_t PFitter::ExecuteSave(Bool_t firstSave)
   // write correlation matrix
   fout << std::endl << " CORRELATION COEFFICIENTS";
   fout << std::endl << "-------------------------------------------------------------------------";
-  if (mnState.HasGlobalCC() && mnState.HasCovariance()) {
+  if (mnState.IsValid() && mnState.HasCovariance()) {
     ROOT::Minuit2::MnGlobalCorrelationCoeff corr = mnState.GlobalCC();
     ROOT::Minuit2::MnUserCovariance cov = mnState.Covariance();
     PIntVector parNo;