[RDF] Keep RJittedDefine pointers invariant when columns don't change.

In order to not write a column multiple times in a snapshot with
variations, the RDefineBase pointer needs to stay invariant when a
variation is generated.
However, the RJittedDefine was returning the pointer to its member, so
it looked like the column was affected by variations. Here, this is
changed to returning "this" when no variations are in effect.

Author: hageboeck

tree/dataframe/src/RJittedDefine.cxx

@@ -66,5 +66,10 @@ void RJittedDefine::MakeVariations(const std::vector<std::string> &variations)
 RDefineBase &RJittedDefine::GetVariedDefine(const std::string &variationName)
 {
    assert(fConcreteDefine != nullptr);
-   return fConcreteDefine->GetVariedDefine(variationName);
+
+   auto &variedDefine = fConcreteDefine->GetVariedDefine(variationName);
+   if (&variedDefine == fConcreteDefine.get())
+      return *this; // Ensures that the pointer is the same across all variations
+   else
+      return variedDefine;
 }