Some fixes...

[jussipo]

I think that to
func valid_frame_received should add fix:
case ACK:
...
if(num_acked <= num_in_window && self->transport_fifo.n_frames) { // There is one frame at least
...

Also to func min_poll one fix:
...
else {
// Sender cannot send new frames so resend old ones (if there's anyone there)
if((window_size > 0) && remote_connected && self->transport_fifo.n_frames) { //There is at least one frame

Better to be safe than sorry...