fix incorrect calculation that led to EINVAL being returned on linux
fix incorrect calculation that led to EINVAL being returned on linux

cwyang/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,yannick/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,yannick/h2o,h2o/h2o,yannick/h2o,cwyang/h2o