Validate REntry belonging to right model when reading

[hahnjo]

Explain what you would like to see improved and how.

When passing a mismatched REntry to RNTupleReader, the following crashes:

#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>

using ROOT::Experimental::REntry;
using ROOT::Experimental::RNTupleModel;
using ROOT::Experimental::RNTupleReader;
using ROOT::Experimental::RNTupleWriter;

#include <cstdint>

void reader_mismatch() {
  auto model = RNTupleModel::CreateBare();
  model->MakeField<std::uint16_t>("u16");
  model->Freeze();

  auto entry = model->CreateEntry();

  {
    auto writer = RNTupleWriter::Recreate(model->Clone(), "ntpl", "mismatch.root");
    // For writing, the entry is checked
    // writer->Fill(*entry);
    auto writeEntry = writer->CreateEntry();
    writer->Fill(*writeEntry);
  }

  {
    auto reader = RNTupleReader::Open(model->Clone(), "ntpl", "mismatch.root");
    // For reading, the entry is NOT checked and results in a crash!
    reader->LoadEntry(0, *entry);
    auto readEntry = reader->GetModel().CreateEntry();
    reader->LoadEntry(0, *readEntry);
  }
}

It would be nicer to throw an exception, as we do for writing. What needs thinking here is if and how this applies to the RNTupleProcessor? @enirolf

ROOT version

master

Installation method

from source

Operating system

any

Additional context

No response