[hist] avoid memleak when SetHistogram, mention transferred ownership (root-project#20254)

[hist] avoid memleak when SetHistogram, mention transferred ownership

Fixes root-project#20252

Author: ferdymercury

hist/hist/inc/TGraph.h

@@ -182,11 +182,11 @@ class TGraph : public TNamed, public TAttLine, public TAttFill, public TAttMarke
    virtual void          Scale(Double_t c1=1., Option_t *option="y"); // *MENU*
    virtual void          SetEditable(Bool_t editable=kTRUE); // *TOGGLE* *GETTER=GetEditable
    virtual void          SetHighlight(Bool_t set = kTRUE); // *TOGGLE* *GETTER=IsHighlight
-   virtual void          SetHistogram(TH1F *h) {fHistogram = h;}
+   virtual void          SetHistogram(TH1F *h);
    virtual void          SetMaximum(Double_t maximum=-1111); // *MENU*
    virtual void          SetMinimum(Double_t minimum=-1111); // *MENU*
    virtual void          Set(Int_t n);
-   virtual void SetOption(Option_t *option = " ") { fOption = option; }
+   virtual void          SetOption(Option_t *option = " ") { fOption = option; }
    virtual void          SetPoint(Int_t i, Double_t x, Double_t y);
    virtual void          SetPointX(Int_t i, Double_t x);
    virtual void          SetPointY(Int_t i, Double_t y);

hist/hist/src/TGraph.cxx

@@ -2266,6 +2266,13 @@ void TGraph::SetHighlight(Bool_t set)
    painter->SetHighlight(this);
 }
 
+/// Set the histogram underlying the TGraph. This transfers the ownership of h to the TGraph. The preexisting fHistogram will be deleted.
+void  TGraph::SetHistogram(TH1F *h)
+{
+   delete fHistogram;
+   fHistogram = h;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Set the maximum of the graph.