Do not even try to keep going on a socket with socklen==0

Back in #1240, r1eo linked to information about how this could happen
with older Linux kernels in response to nmap.  Bugs #4545 and #4547
are about how our approach to trying to deal with this condition was
broken and stupid.  Thanks to wanoskarnet for reminding us about #1240.

This is a fix for the abovementioned bugs, and is a bugfix on
0.1.0.3-rc.