Geary.Util.Source: Fix GLib `old_val > 0` critical
If a source object prints a log message from a destructor the engine's logging hander will attempt to ref it despite the object bing finalised, resulting in the critical.
Fixes #650 (closed)
If a source object prints a log message from a destructor the engine's logging hander will attempt to ref it despite the object bing finalised, resulting in the critical.
Fixes #650 (closed)