[tree] Force calling GetNdata before EvalInstance in BuildIndex

As recommended in https://github.com/root-project/root/pull/18766/files

Fixes #11883

Author: ferdymercury

tree/treeplayer/src/TTreeIndex.cxx

@@ -11,6 +11,8 @@
 
 /** \class TTreeIndex
 A Tree Index with majorname and minorname.
+
+If minorname is "0", just majorname will be used.
 */
 
 #include "TTreeIndex.h"
@@ -181,6 +183,14 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
          fMajorFormula->UpdateFormulaLeaves();
          fMinorFormula->UpdateFormulaLeaves();
       }
+      if ((fMajorFormula->GetNdata() + fMinorFormula->GetNdata()) <= 0) {
+         // Calling GetNdata is essential before calling EvalInstance, otherwise a wrong
+         // result is silently returned by EvalInstance below if formula is value from a variable-sized array
+         // We raise an error to prevent the if clause being optimized-out if we do not use the return
+         Error("TTreeIndex",
+                 "In tree entry %lld, Ndata in formula is zero for both '%s' and '%s'", i,
+                 fMajorName.Data(), fMinorName.Data());
+      }
       auto GetAndRangeCheck = [this](bool isMajor, Long64_t entry) {
          LongDouble_t ret = (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<LongDouble_t>();
          // Check whether the value (vs significant bits) of ldRet can represent