gdm/util: Make sure error is GError before checking it

The fingerprint device fetching code has a generic error handler,
that assumes the passed in error is GError. If it's not a
GError it will fail trying to use GError specific methods.

This commit adds some validation checking.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3319>

js/gdm/util.js

@@ -395,15 +395,16 @@ export class ShellUserVerifier extends Signals.EventEmitter {
     _handleFingerprintError(e) {
         this._fingerprintReaderType = FingerprintReaderType.NONE;
 
+        if (e instanceof GLib.Error) {
             if (e.matches(Gio.IOErrorEnum, Gio.IOErrorEnum.CANCELLED))
                 return;
             if (e.matches(Gio.DBusError, Gio.DBusError.SERVICE_UNKNOWN))
                 return;
-
             if (Gio.DBusError.is_remote_error(e) &&
                 Gio.DBusError.get_remote_error(e) ===
                     'net.reactivated.Fprint.Error.NoSuchDevice')
                 return;
+        }
 
         logError(e, 'Failed to interact with fprintd service');
     }