rt_frtchord: fix fd subscriptions
rt_frtchord: fix fd subscriptions

In certain cases, warnings on unsubscriptions for non-existing subscriptions where thrown.
This commit should fix those warnings.
caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,jvf/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris