MFC r344428:

This patch addresses an issue brought up by bz@ in D18968:
When TCP_REASS_LOGGING is defined, a NULL pointer dereference would happen,
if user data was received during the TCP handshake and BB logging is used.

A KASSERT is also added to detect tcp_reass() calls with illegal parameter
combinations.

Reported by: bz@
Reviewed by: rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19254