[RF] Don't derefernce pointers in code emitted by RooFit codegen

Differentiating through Cling runtime checks in pointer dereferncing can
result in errors:
```txt
warning: attempted differentiation of function 'cling_runtime_internal_throwIfInvalidPointer' without definition and no suitable overload was found in namespace 'custom_derivatives'
note: numerical differentiation is not viable for 'cling_runtime_internal_throwIfInvalidPointer'; considering 'cling_runtime_internal_throwIfInvalidPointer' as 0
<<< cling interactive line includer >>>:1:1: error: read-only variable is not assignable

^
```

Author: guitargeek

roofit/codegen/src/CodegenImpl.cxx

@@ -102,7 +102,7 @@ void rooHistTranslateImpl(RooAbsArg const &arg, CodegenContext &ctx, int intOrde
    }
    std::string const &offset = dataHist.calculateTreeIndexForCodeSquash(ctx, obs);
    std::string weightArr = dataHist.declWeightArrayForCodeSquash(ctx, correctForBinSize);
-   ctx.addResult(&arg, "*(" + weightArr + " + " + offset + ")");
+   ctx.addResult(&arg, "(" + weightArr + ")[" + offset + "]");
 }
 
 std::string realSumPdfTranslateImpl(CodegenContext &ctx, RooAbsArg const &arg, RooArgList const &funcList,

roofit/roofitcore/src/RooDataHist.cxx

@@ -1025,14 +1025,16 @@ std::string RooDataHist::calculateTreeIndexForCodeSquash(RooFit::Experimental::C
          return "";
       }
 
-      code += " + " + binning->translateBinNumber(ctx, *theVar, idxMult);
+      if (i > 0)
+         code += " + ";
+      code += binning->translateBinNumber(ctx, *theVar, idxMult);
 
       // Use RooAbsLValue here because it also generalized to categories, which
       // is useful in the future. dynamic_cast because it's a cross-cast.
       idxMult *= dynamic_cast<RooAbsLValue const *>(internalVar)->numBins();
    }
 
-   return "(" + code + ")";
+   return _vars.size() == 1 ? code : "(" + code + ")";
 }
 
 ////////////////////////////////////////////////////////////////////////////////