TCP: move Windows Scaling and SCPS checks after options decoding

This was changed in g8881f3f in an attempt to avoid throwing an exception when decoding the options field

Bug: 10514
Change-Id: Ia4b49f484d6255090c5a6e425a9716b48ccc4cb5
Reviewed-on: https://code.wireshark.org/review/5495
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>