tcp: remove goto and address another NULL deref in SACK

Missed another NULL dereference during KASSERTS after traversing
the scoreboard. While at it, scratch the goto by making the
traversal conditional, and remove duplicate checks using an
unconditional loop with all checks inside.

Reviewed By: hselasky
PR: 263445
MFC after: 1 week
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D35428