MINOR: flt_spoe: deinit spoe agent proxy upon agent release

Even though spoe agent proxy is statically allocated, it uses the proxy
API and is initialized like a regular proxy, thus specific cleanup is
required upon release. This is not tagged as a bug because as of now this
would only cause some minor memory leak upon deinit.

We check the presence of proxy->id to know if it was initialized since
we cannot rely on a pointer for that.

src/flt_spoe.c

@@ -321,6 +321,8 @@ static void spoe_release_agent(struct spoe_agent *agent)
 	}
 	free(agent->events);
 	free(agent->engine_id);
+	if (agent->fe.id)
+		deinit_proxy(&agent->fe);
 	free(agent);
 }