Guard against libwebsockets assert in 3.2.1 and up.

src/mux_epoll.c

@@ -242,6 +242,10 @@ static void loop_handle_reads_writes(struct mosquitto *context, uint32_t events)
 	socklen_t len;
 	int rc;
 
+	if(context->sock == INVALID_SOCKET){
+		return;
+	}
+
 #ifdef WITH_WEBSOCKETS
 	if(context->wsi){
 		struct lws_pollfd wspoll;

src/mux_poll.c

@@ -246,6 +246,10 @@ static void loop_handle_reads_writes(struct pollfd *pollfds)
 			continue;
 		}
 
+		if(pollfds[context->pollfd_index].fd == INVALID_SOCKET){
+			continue;
+		}
+
 		assert(pollfds[context->pollfd_index].fd == context->sock);
 
 #ifdef WITH_WEBSOCKETS