final fix for CVE-2004-0230

This (again) relates to #14
Last two fixes, I used the wrong if statement (ackno == pcb->snd_nxt) which is only relevant for the SYN_SENT special case, not the normal ESTABLISHED state.
In that case, the seqno has to match the expected one.
If not, an ACK is sent (unmodified not like the last change)

This creates the correct and expected behavior in my lab setup and still fixes the vulnerability that CVE-2004-0230 describes