fix a crash on qpack.ctx, amending h2o/h2o#2473
fix a crash on qpack.ctx, amending h2o/h2o#2473

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