placesview: plug some memory leaks

The GFile containing the Computer item was not
properly dereferenced, so plug that leak by both
dereferencing it and adding some reference management
on GtkPlacesViewRow.

gtk/gtkplacesview.c

@@ -874,6 +874,9 @@ add_computer (GtkPlacesView *view)
                       NULL);
 
   insert_row (view, row, FALSE);
+
+  g_object_unref (icon);
+  g_object_unref (file);
 }
 
 static void