tcp: remove goto and address another NULL deref in SACK

[rscheff]

Summary

Missed another NULL dereference during KASSERTS after traversing
the scoreboard. While at it, scratch the goto by making the
traversal conditional.

[• hselasky]

sys/netinet/tcp_sack.c

969

The NULL check should be inside the if (SEQ_GEQ()).

[• hselasky]

sys/netinet/tcp_sack.c

962

I would just write the code like this:

for (;;) {
hole = TAILQ_NEXT(hole, scblink);
if (hole == NULL)
return (hole);
if ....
   break;
}

962

To avoid duplicated NULL checks.

[rscheff]

• unconditional loop, with checks inside

[• hselasky]

Looks good!