[tree] Always inform index formulas about changed tree in friend chain

To avoid following valgrind error leading to crashes in multithreaded RDF runs:

```
==735217== Invalid read of size 8
==735217==    at 0x6A4517F: TTreeFormula::UpdateFormulaLeaves() (TTreeFormula.cxx:5121)
==735217==    by 0x6A5B9AF: TTreeIndex::UpdateFormulaLeaves(TTree const*) (TTreeIndex.cxx:625)
==735217==    by 0x67A5447: TChain::LoadTree(long long) (TChain.cxx:1670)
==735217==    by 0x6A94DBD: TTreeReader::SetEntryBase(long long, bool) (TTreeReader.cxx:669)
==735217==    by 0x6A96027: TTreeReader::SetEntry(long long) (TTreeReader.h:225)
==735217==    by 0x7576BAA: TTreeReader::Next() (TTreeReader.h:217)
==735217==    by 0x756D591: (anonymous namespace)::validTTreeReaderRead(TTreeReader&) (RLoopManager.cxx:538)
==735217==    by 0x756D9A9: ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}::operator()(TTreeReader&) const (RLoopManager.cxx:575)
==735217==    by 0x7574F7E: void std::__invoke_impl<void, ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}&, TTreeReader&>(std::__invoke_other, ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}&, TTreeReader&) (invoke.h:61)
==735217==    by 0x757480E: std::enable_if<is_invocable_r_v<void, ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}&, TTreeReader&>, void>::type std::__invoke_r<void, ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}&, TTreeReader&>(ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}&, TTreeReader&) (invoke.h:111)
==735217==    by 0x757433A: std::_Function_handler<void (TTreeReader&), ROOT::Detail::RDF::RLoopManager::RunTreeProcessorMT()::{lambda(TTreeReader&)#1}>::_M_invoke(std::_Any_data const&, TTreeReader&) (std_function.h:290)
==735217==    by 0x6ABCE3E: std::function<void (TTreeReader&)>::operator()(TTreeReader&) const (std_function.h:591)
==735217==  Address 0x15b4e000 is 0 bytes inside a block of size 712 free'd
==735217==    at 0x48456C6: operator delete(void*) (vg_replace_malloc.c:1131)
==735217==    by 0x4E9494B: TStorage::ObjectDealloc(void*) (TStorage.cxx:325)
==735217==    by 0x4E6B181: TObject::operator delete(void*) (TObject.cxx:1113)
==735217==    by 0x67F47E9: TTree::~TTree() (TTree.cxx:1023)
==735217==    by 0x4EED8F1: TCollection::GarbageCollect(TObject*) (TCollection.cxx:736)
==735217==    by 0x4EF5D4C: TList::Delete(char const*) (TList.cxx:535)
==735217==    by 0x4EF0428: THashList::Delete(char const*) (THashList.cxx:215)
==735217==    by 0x5507848: TDirectoryFile::Close(char const*) (TDirectoryFile.cxx:585)
==735217==    by 0x5523835: TFile::Close(char const*) (TFile.cxx:986)
==735217==    by 0x5521226: TFile::~TFile() (TFile.cxx:563)
==735217==    by 0x55215FF: TFile::~TFile() (TFile.cxx:600)
==735217==    by 0x67A4990: TChain::LoadTree(long long) (TChain.cxx:1490)
==735217==  Block was alloc'd at
==735217==    at 0x4841FEC: operator new(unsigned long) (vg_replace_malloc.c:487)
==735217==    by 0x4E948DF: TStorage::ObjectAlloc(unsigned long) (TStorage.cxx:293)
==735217==    by 0x48C648E: TObject::operator new(unsigned long) (TObject.h:181)
==735217==    by 0x672E04D: ROOT::new_TTree(void*) (G__Tree.cxx:4363)
==735217==    by 0x4F4AA25: TClass::NewObject(TClass::ENewType, bool) const (TClass.cxx:5084)
==735217==    by 0x4F4A932: TClass::New(TClass::ENewType, bool) const (TClass.cxx:5061)
==735217==    by 0x556146A: TKey::ReadObj() (TKey.cxx:809)
==735217==    by 0x55091D3: TDirectoryFile::Get(char const*) (TDirectoryFile.cxx:988)
==735217==    by 0x67A4CA4: TChain::LoadTree(long long) (TChain.cxx:1542)
==735217==    by 0x6A94DBD: TTreeReader::SetEntryBase(long long, bool) (TTreeReader.cxx:669)
==735217==    by 0x6A96027: TTreeReader::SetEntry(long long) (TTreeReader.h:225)
==735217==    by 0x6A94855: TTreeReader::SetEntriesRange(long long, long long) (TTreeReader.cxx:560)
```

tree/tree/src/TChain.cxx

@@ -1667,7 +1667,7 @@ Long64_t TChain::LoadTree(Long64_t entry)
          TTree* t = fe->GetTree();
          if (!t) continue;
          if (t->GetTreeIndex()) {
-            t->GetTreeIndex()->UpdateFormulaLeaves(nullptr);
+            t->GetTreeIndex()->UpdateFormulaLeaves(GetTree());
          }
          if (t->GetTree() && t->GetTree()->GetTreeIndex()) {
             t->GetTree()->GetTreeIndex()->UpdateFormulaLeaves(GetTree());