Fix KASSERT during tcp_newtcpcb when low on memory

While testing with system default cc set to cubic, and
running a memory exhaustion validation, FreeBSD panics for a
missing inpcb reference / lock.

Reviewed by: rgrimes (mentor), tuexen (mentor)
Approved by: rgrimes (mentor), tuexen (mentor)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D25583