object-cache: Avoid use after free of task error

The error should not be touched after passing it to
`g_task_return_error()` as noted by its documentation:

  Note that since the task takes ownership of @error, and since the
  task may be completed before returning from g_task_return_error(),
  you cannot assume that @error is still valid after calling this.

However, previously, the code did try to free error since the
`local_error` was defined with `g_autoptr(GError)`.


(cherry picked from commit f00fd653cb)

shell/cc-object-storage.c

@@ -91,7 +91,7 @@ create_dbus_proxy_in_thread_cb (GTask        *task,
 
   if (local_error)
     {
-      g_task_return_error (task, local_error);
+      g_task_return_error (task, g_steal_pointer (&local_error));
       return;
     }