Incorrect handling of partially loaded chains in TChain::Add(TChain* chain)

[pcanal]

This problem has been reported at https://root-forum.cern.ch/t/tchain-add-tchain-leaves-getentries-reporting-a-wrong-too-small-value/64904
and can be reproduced chain_add_entries.C
with the incorrect output being similar to:

ROOT version: 6.40.02

entries written:   A=501  B=1055  C=806   →  true total = 2362
per-chain GetEntries(): a=501  b=1055  c=806   (sum=2362)

after  a->Add(b); a->Add(c):
  GetEntries()     = 489   ← WRONG (expected 2362)
  GetEntriesFast() = 489   ← WRONG
  files in chain   = 16   (all present)
  reachable via LoadTree loop = 2362   ← correct total

single chain (Add per file): GetEntries() = 2362   ← correct

It is known to be broken in 6.40.02, 6.38.04 but correct in 6.38.02 and older.

[pcanal]

The problem was probably uncovered with the change in 71c6df489874fb90e45b12b00360b5fb7b8ab34c. However the problem was pre-existing in the code.