xios: null-guard CXios::globalRegistry deref in CContext::finalize (attached-mode SIGSEGV)

[JanStreffing]

Summary

Fixes a SIGSEGV at xios_context_finalize for FESOM_WITH_XIOS=ON runs in attached mode (using_server=false), discovered while bringing up the new fesom2_xios.yml CI run-test on FESOM/fesom2.

In src/node/context.cpp's CContext::finalize, the "Mode attache" branch dereferences CXios::globalRegistry unconditionally:

if (server->intraCommRank==0) CXios::globalRegistry->mergeRegistry(*registryOut) ;

CXios::globalRegistry is only allocated on client rank 0 (cxios.cpp:112) but the merge is gated on server->intraCommRank==0; in attached mode those two ranks need not coincide, so a non-rank-0 process hits the deref on a null pointer and SIGSEGVs in xios::CRegistry::mergeRegistry. Files were already flushed by closeAllFile() earlier in the same branch, so the crash leaves the model output intact but the process exits with status 139.

The fix is a one-token addition of a null check; the patched line is equivalent for server mode (globalRegistry is always non-null on the server's rank 0) and lets attached-mode runs exit cleanly. Applied to both fesom2_ci/xios/ and fesom2_test_refactoring/xios/ source trees.

Test plan

• Verified locally on Levante (intel-oneapi-compilers/2022.0.1 + openmpi/4.1.2 + netcdf-c/4.8.1, 2-rank pi mesh, 6 XIOS-output fields sst/a_ice/temp/salt/u/v): without patch the run produces all 6 netcdf files but exits 139 with Caught signal 11 in xios::CRegistry::mergeRegistry; with patch the run produces the same 6 netcdf files and exits cleanly with fesom should stop with exit status = 0.
• docker-publish.yml rebuilds both fesom2_ci-master and fesom2_test_refactoring-master.
• FESOM/fesom2 PR #902's fesom2_xios.yml workflow passes end-to-end against the new images.

Notes

• The patch is identical to one carried in some downstream XIOS forks. We could upstream it to the IPSL XIOS svn separately.
• fesom2_ci/xios/ and fesom2_test_refactoring/xios/ carry duplicated source today (per the note in fesom2_test_refactoring: add XIOS 2.5 at /xios for upcoming xios run-test #5). A future cleanup that moves the XIOS source to a shared repo-root _shared/xios/ will collapse this two-file change to one.