tcp rack: fix memory corruption

When in rack_output() jumping to the label out, don't write errno into
the log buffer, since the pointer is not initialized.

Reported by: Coverity Scan
CID: 1523773
Reviewed by: rscheff
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44647