tcp: Rack can crash with the new non-TSO fix..

Turns out the location of the check to see if we can do output is in the wrong place. We need
to jump off to the compressed acks before handling that case since th is NULL in the
compressed ack case which is handled differently anyway.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39690