[tree] add unit test for ttreereadervaluefast bug

See https://its.cern.ch/jira/browse/ROOT-8842

Author: ferdymercury

tree/treeplayer/test/regressions.cxx

@@ -10,6 +10,8 @@
 #include "TString.h"
 #include "TLorentzVector.h"
 #include <ROOT/TestSupport.hxx>
+#include "ROOT/TTreeReaderValueFast.hxx"
+#include "TNtuple.h"
 
 #include "gtest/gtest.h"
 
@@ -263,3 +265,20 @@ TEST(TTreeFormulaRegressions, WrongName)
       EXPECT_EQ(t.Draw("s.Eta()", ""), -1);
    }
 }
+
+// ROOT-8842
+TEST(TTreeReaderRegressions, ValueFastTuple)
+{
+   TNtuple tree("tuple", "ROOT-8842", "px:py:pz:energy");
+   for(auto i=0; i<1000000; ++i)
+      tree.Fill(i,i+1,i+2,i+3);
+   ROOT::Experimental::TTreeReaderFast reader(&tree);
+   ROOT::Experimental::TTreeReaderValueFast<float> px(reader, "px");
+   ROOT::Experimental::TTreeReaderValueFast<float> py(reader, "py");
+   ROOT::Experimental::TTreeReaderValueFast<float> pz(reader, "pz");
+   double total = 0.0;
+   // reader.SetEntry(0); If I uncomment this, the crash disappears
+   for (auto it = reader.begin();  it != reader.end();  ++it)
+      total += sqrt((*px)*(*px) + (*py)*(*py) + (*pz)*(*pz));
+   EXPECT_EQ(total, 8.6602627e+11);
+}