fix broken listen socket piggybacking

As reported here

http://libwebsockets.org/trac/ticket/11

the code for connection storm handling had rotted, fds[0] is no longer
always related to the listen socket when serving.

This patch updates it to determine the listen socket correctly to stop
infinite recursion here.

Reported-by: amn
Signed-off-by: Andy Green <andy.green@linaro.org>