[http2] Fix crash in priority demotion handling
[http2] Fix crash in priority demotion handling

When determining if priority demotion via priority header has
happened, correctly consider a case where `h2o_http2_conn_get_stream`
returns NULL.

This can happen when a parent stream is actually closed beforehand.

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