fuzzer: fix init time race
fuzzer: fix init time race

This fixes a race between upstream_thread and sending a request.
It's possible that a request is injected via `created_accepted` before
the socket pair in `upstream_thread` is created.
If that request causes h2o to close the connection, and the socketpair
is created in the interm, it's possible that the socketpair gets the
same fd number as the closed request.
This means that the `is_valid_fd` test down the line doesn't fail,
resulting in a timeout.

yannick/h2o,devnexen/h2o,deweerdt/h2o,lkwg82/h2o,cwyang/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o,lkwg82/h2o,deweerdt/h2o,yannick/h2o,yannick/h2o,lkwg82/h2o,deweerdt/h2o,cwyang/h2o,cwyang/h2o,h2o/h2o,devnexen/h2o,lkwg82/h2o,lkwg82/h2o,lkwg82/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,deweerdt/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,deweerdt/h2o,cwyang/h2o,deweerdt/h2o,deweerdt/h2o,lkwg82/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,deweerdt/h2o,h2o/h2o,h2o/h2o,h2o/h2o,yannick/h2o,lkwg82/h2o,devnexen/h2o,h2o/h2o,deweerdt/h2o,yannick/h2o,lkwg82/h2o