crashdb: make _duplicate_db_sync_status more readable

[bdrung]

Coverity 2023.9.2 rule COV_PY_COPY_PASTE_ERROR_PYTHON complains about real_fixed_version in CrashDatabase.duplicate_db_fixed looks like a copy-paste error.

For the "crash got reopened" case, real_fixed_version is None and therefore the call to duplicate_db_fixed sets the fixed version back to 'still unfixed'. Make that code more readable by explicitly passing None to duplicate_db_fixed.