Make TNetXNGFile::Close() reentrant

[linev]

Check if fFile pointer still available

Fixes #13503

[phsft-bot]

Starting build on ROOT-performance-centos8-multicore/soversion, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac11/noimt, mac12arm/cxx20, windows10/default
How to customize builds

[phsft-bot]

Build failed on ROOT-ubuntu2204/nortcxxmod.
Running on root-ubuntu-2204-1.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.runtutorials.test_import_numba
• projectroot.bindings.pyroot.pythonizations.test.test_import_numba
• projectroot.bindings.pyroot.pythonizations.test.pyunittests_pyroot_dependency_versions
• projectroot.roottest.python.numba.roottest_python_numba_numba

[phsft-bot]

Build failed on ROOT-ubuntu2004/python3.
Running on root-ubuntu-2004-1.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.distrdf.dask.roottest_python_distrdf_dask_test_all

[phsft-bot]

Build failed on mac11/noimt.
Running on macphsft20.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.distrdf.dask.roottest_python_distrdf_dask_test_all

[github-actions]

Test Results

       10 files         10 suites   2d 6h 25m 18s ⏱️
  2 481 tests      495 ✔️   0 💤 1 986 ❌
23 753 runs  21 456 ✔️ 81 💤 2 216 ❌

For more details on these failures, see this check.

Results for commit 6c18c77.

[vepadulano]

@linev could you also add a test along the lines of #13503 please?

[linev]

@vepadulano Do we have xrootd server permanently running with test file?

[vepadulano]

Sure, you can choose any file from

[root_src]$: grep -r "root://eospublic.cern.ch/"

[linev]

@vepadulano Can you approve #13562