gs-appstream: Fix possible NULL GError dereference under gs_appstream_gather_merge_data()
The passed-in GCancellable can be cancelled after the XbBuilder checks for its state, causing a valid XbSilo returned with no GError set, which can result in a NULL GError dereference in the debug message, thus do not test whether the GCancellable is cancelled or not, trust the XbBuilder whether it returned correct data or not.
Closes downstream https://bugzilla.redhat.com/show_bug.cgi?id=2265824