tcp_io: save read_queue to local var before queue_data_put, add NULL guard after

lib/tcp_io.c

@@ -209,8 +209,10 @@ static void read_cb(socket_t sock, void* arg) {
     if (n > 0) {
         e->dgram = buf;
         e->len = (uint16_t)n;
-        queue_data_put(tc->read_queue, e);
-        if (tc->read_queue->count >= tc->read_high_water && !tc->read_paused) {
+        struct ll_queue* rq = tc->read_queue;
+        queue_data_put(rq, e);
+        // tc мог быть уничтожен через callback chain — read_queue обнулён
+        if (rq && tc->read_queue && tc->read_queue->count >= tc->read_high_water && !tc->read_paused) {
             uasync_set_socket_read(tc->ua, tc->socket_id, 0);
             tc->read_paused = 1;
             queue_waiter_wait(tc->read_queue, &tc->read_waiter, resume_read_cb, tc);