[tree] Fix interaction between GetEntries and GetListOfFriends

[vepadulano]

Fixes #17820

Full analysis of the issue follows:

1. TChain::GetEntries calls TChain::LoadTree(TTree::kMaxEntries - 1)

2. Internally, this will set fReadEntry == -2, due to how LoadTree works and the fact it's trying to read beyond the last entry in the chain

3. This has an effect here

   root/tree/tree/src/TChain.cxx

   Lines 1675 to 1679 in f33985d

   	if (treeReadEntry == -2) {
   	// an entry after the end of the chain was requested (it usually happens when GetEntries is called)
   	t->LoadTree(entry);
   	} else {
   	t->LoadTreeFriend(entry, this);

   where t is the current friend being traversed in the list of friends of the chain. We call t->LoadTree(TTree::kMaxEntries - 1).

4. Inside the call to LoadTree on the friend, we reach this line

   root/tree/tree/src/TChain.cxx

   Line 1484 in f33985d

   InvalidateCurrentTree();

   where the fTree data member of the friend is invalidated.

5. Back to the LoadTree of the main chain, these lines

   root/tree/tree/src/TChain.cxx

   Lines 1681 to 1684 in f33985d

   	TTree* friend_t = t->GetTree();
   	if (friend_t) {
   	auto localfe = fTree->AddFriend(t, fe->GetName());
   	localfe->SetBit(TFriendElement::kFromChain);

   would normally connect the fTree data member of the main chain to the current friend, so the list of friends is properly populated. But due to 4., friend->GetTree() will return nullptr so the connection won't happen.

This commit fixes the issue by always calling LoadTreeFriend on the friend, which will anyway call LoadTree eventually but with the index value and does not incur in the wrong mechanics.

[vepadulano]

Note that the current change apparently fixed the attached reproducer on my local machine, but evidently something is still missing. Suggestions on how to proceed are welcome!