Fix a race condition on tcp listen socket teardown with pending
connections in the accept queue and contiguous new incoming SYNs.

Compared to the original submitters patch I've moved the test
next to the SYN handling to have it together in a logical unit
and reworded the comment explaining the issue.

Submitted by: Matt Miller <matt@matthewjmiller.net>
Submitted by: Juan Mojica <jmojica@gmail.com>
Reviewed by: Matt Miller (changes)
Tested by: pho
MFC after: 1 week