BUG/MINOR: sink: free the forwarding task on exit

ASAN reported a small leak of the sink's forwarding task on exit.

This should be backported as far as 2.2.

src/sink.c

@@ -1404,6 +1404,7 @@ static void sink_deinit()
 				ring_free(sink->ctx.ring);
 		}
 		LIST_DELETE(&sink->sink_list);
+		task_destroy(sink->forward_task);
 		free(sink->name);
 		free(sink->desc);
 		free(sink);