pfsync: reject invalid SCTP states

SCTP states should always have a src scrub object associated with them.
Crafted pfsync packets might not have this, leading to us derferencing a
NULL pointer on cleanup.

Validate the pfsync state insertion packet to make sure this is correct.

PR: 294989
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit bf6d00afdb6171ba04a1c5a7fde904cde87d212d)