tcp: rejects FIN+SYN packets as invalid

tcp: rejects FIN+SYN packets as invalid

Ticket: #4569

If a FIN+SYN packet is sent, the destination may keep the
connection alive instead of starting to close it.
In this case, a later SYN packet will be ignored by the
destination.

Previously, Suricata considered this a session reuse, and thus
used the sequence number of the last SYN packet, instead of
using the one of the live connection, leading to evasion.

This commit errors on FIN+SYN so that they do not get
processed as regular FIN packets.