[DF] RVecs of non-split branches can read from invalid addresses

When reading a TTreeReaderArray<Object> as an RVec<Object> in RDataFrame, we need to decide whether the RVec can just be a view over a contiguous slab of memory or, in case TTreeReaderArray elements are not contiguous in memory, we need a deep copy of the TTreeReaderArray contents (RVec assumes contiguousness, as it is backed by std::vector).

The decision is based on a check on the addresses of first and second elements of the TTreeReaderArray (deferred to the first event that has at least two elements in the array). The pointer arithmetic to check for contiguousness is currently wrong.