TCP RACK, BBR: ensure return value is always ininitialized

Do not return an uninitialized value from ctf_do_queued_segments()
in case no packets are actually processed (all are skipped).

Reviewed by: rrs
CID: 1523774
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48217