[DF] Regression in recognition of nested branch names

   struct c_t{

     double a;

     double b;

   };

   void p1_read()

   {

     struct c_t c;

     c.a = 1;

     c.b = 2;

     TFile * f1 = new TFile("f1.root","RECREATE");

     TTree * t1 = new TTree("t1", "t1");

     t1->Branch("c",&c);

     t1->Fill();

     t1->Fill();

     t1->Write();

     f1->Close();

     ROOT::RDataFrame aData("t1", "f1.root");

     aData.Display("a")->Print();

   }