fix panic when rescue retransmission and FIN overlap

PR: 254244
PR: 254309
Reviewed By: transport, hselasky, tuexen
Approved by: re (cperciva)
MFC after: immediately
Sponsored By: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29315

(cherry picked from commit e9f029831fa5747ae1b405f5716c52cb4ebf1e04)