Fix duplicate GError use when "uid" is missing
    
Some GLib versions complain loudly about this.
    
To reproduce, call e.g. RegisterAuthenticationAgent with the following parameters:
("unix-process", {"pid": __import__('gi.repository.GLib', globals(), locals(), ['Variant']).Variant("u", 1), "start-time": __import__('gi.repository.GLib', globals(), locals(), ['Variant']).Variant("t", 1)}), "cs", "/"

Then lookup_asv will set the error object within server_handle_register_authentication_agent but won’t fail, and later error paths will reset the GError with a new error value.