Prevent an assertion if proceed_request needs to send an error while the http2 connection is shutting down.
Prevent an assertion if proceed_request needs to send an error while the http2 connection is shutting down.

Here is the path that's causing this:
```
    stream_send_error lib/http2/connection.c
    proceed_request lib/http2/connection.c
    on_head lib/core/proxy.c
    on_error lib/common/http1client.c
    on_send_timeout lib/common/http1client.c
```

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