progress-info: Prevent abortion caused by missing locks

The test-file-operations-move-files and test-file-operations-copy-files test cases randomly crash with "GLib-FATAL-CRITICAL: g_source_is_destroyed: assertion 'g_atomic_int_get (&source->ref_count) > 0' failed", which causes GitLab CI pipeline failures among others, which is a shame.

I am not able to reproduce those crashes locally, so I can't easily verify the call stack, but only place where g_source_destroy() is called is when handling progress infos. I see two potential issues which might cause this. Let's try to fix them.