server/red_channel: do not attempt to write if the channel client is disconnected

The red_channel_client_event call to red_channel_client_receive might result
in a disconnected channel client. The following call to
red_channel_client_push may call to red_peer_handle_outgoing with a
disconnected socket.

server/red_channel.c

@@ -305,6 +305,10 @@ static void red_peer_handle_outgoing(RedsStream *stream, OutgoingHandler *handle
 {
     ssize_t n;
 
+    if (!stream) {
+        return;
+    }
+
     if (handler->size == 0) {
         handler->vec = handler->vec_buf;
         handler->size = handler->cb->get_msg_size(handler->opaque);