[tree] avoid resetting index in entry list copy constructor

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://its.cern.ch/jira/browse/ROOT-10807

Checklist:

• tested changes locally
• updated the docs (if necessary)

[pcanal]

Could you add a test?

[github-actions]

Test Results

    19 files      19 suites   3d 13h 4m 57s ⏱️
 2 813 tests  2 812 ✅ 0 💤  1 ❌
51 945 runs  51 934 ✅ 0 💤 11 ❌

For more details on these failures, see this check.

Results for commit 6ad61f8.

♻️ This comment has been updated with latest results.

[ferdymercury]

Test added. I am not sure if the failures are race condition or if it's breaking sth.

[pcanal]

It is a bit surprising changing for the usual starting point to the previously visited entry fixes the problem. Those 2 data members are meant to be accelerator and so the default value should have been fine. ... What is the mechanism leading to the failure?

[ferdymercury]

I guess it was an empirical fix. Without this, the returned value is PreviousVisitedEntry + 1, so 16 even if Index is -1
It seems as if the variable index is -1 but the entry pointer is still at 15.