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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48217

(cherry picked from commit b5739c8b12226c791bc8e7a4ac6e48965333509a)