Problems with `TH1::GetQuantile`

[lmoneta]

The function TH1::GetQuantile is not returning in some case the correct result.

There is already this JIRA issue open: https://sft.its.cern.ch/jira/browse/ROOT-8085

and not it is reported that gives wrong values for 0 and 100 percentiles, see:

https://root-forum.cern.ch/t/th1-getquantiles-gives-wrong-value-for-100-percentile/53284/3

See this example from JIRA:

void quantiles() {
   TCanvas *c1 = new TCanvas();
   c1->Divide(1,2);
   TH1D *h = new TH1D("h","h",10,0,10);
   h->Fill(5);
   c1->cd(1);
   h->Draw();
   Double_t *p = new Double_t[1];
   p[0] = 0.5;
   Double_t *q = new Double_t[1];
   q[0] = 0;
   h->GetQuantiles(1,q,p);

   cout << "Median of h:" << q[0] << std::endl;
   cout << "This is ok!" << endl;

   TH1D *h2 = new TH1D("h2","h2",10,0,10);
   //for (int i = 0; i < 10; ++i) h2->Fill(i); 
   h2->Fill(2);
   h2->Fill(8);
   c1->cd(2);
   h2->Draw();
   Double_t *p2 = new Double_t[1];
   p2[0] = 0.5;
   Double_t *q2 = new Double_t[1];
   q2[0] = 0;
   int nq = h2->GetQuantiles(1,q2,p2);

   cout << "Median of h2 (and is never the same result when you execute the macro several times).: " << q2[0] << "  nq = " << nq << std::endl;
   cout << "NO!!! should be 5.5 as well" << endl;
}