Unexpected corruption

gistfile1.txt

struct netconn *conn = netconn_new_with_proto_and_callback(NETCONN_UDP, 0, NULL);
        char buf[512];

        if(!conn)
        {
            cerr << "Failed to allocate socket";
            abort();
        }

        netconn_bind(conn, IP_ADDR_ANY, 5000);
        netconn_set_recvtimeout(conn, 0); // blocking

        struct netbuf* nbFrom = netbuf_new();
        netconn_recv(conn, &nbFrom);

        void* _recvBuf;
        u16_t recvLen = 0;
        const size_t offset = 50;

        err_t err = netbuf_data(nbFrom, &_recvBuf, &recvLen);

        clog << "Status: " << (uint16_t)err << endl;
        clog << "Got " << recvLen << " bytes" << endl;
        debugPrintBuffer((char*)_recvBuf, recvLen);
        clog << endl;

        memset(buf, 0, sizeof(buf));
        char* offsetbuf = buf + offset;
        sprintf(offsetbuf, "Test send! (got: %d bytes)", recvLen);
        size_t buflen = strlen(offsetbuf);

        struct netbuf* nb = netbuf_new();
        netbuf_ref(nb, offsetbuf, buflen);

        clog << "About to send" << endl;

        clog << "buffer pre offset: ";
        debugPrintBuffer((char*)buf, offset);
        clog << endl;

        netconn_sendto(conn, nb, netbuf_fromaddr(nbFrom), 5001);

        clog << "buffer pre offset: ";
        debugPrintBuffer((char*)buf, offset);
        clog << endl;

        clog << "Sent" << endl;

        netbuf_delete(nb);
        netbuf_delete(nbFrom);
        netconn_disconnect(conn);
        netconn_delete(conn);

Perhaps it's cause I'm not waiting long enough for the TX async to complete sending, as alluded to here http://savannah.nongnu.org/bugs/?44805