device: Better define ownership passing for results

Some things were odd with regard to the ownership of passed objects. Try
to make things sane overall, in particular with the possible floating
FpPrint reference.

libfprint/fpi-image-device.c

@@ -226,7 +226,7 @@ fpi_image_device_minutiae_detected (GObject *source_object, GAsyncResult *res, g
 
           if (fpi_print_bz3_match (template, print, priv->bz3_threshold, &error) == FPI_MATCH_SUCCESS)
             {
-              result = g_object_ref (template);
+              result = template;
               break;
             }
         }