Always unlink pending write requests when entering close_wait
Seen on a stream in `H2O_HTTP3_SERVER_STREAM_STATE_CLOSE_WAIT`:
```
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /build/lib/core/request.c:406:62 in
h2o: /build/lib/http3/server.c:988: void run_delayed(h2o_timer_t *): Assertion `stream->req_body != NULL' failed.
received fatal signal 6
```
@@ -432,6 +432,8 @@ static void set_state(struct st_h2o_http3_server_stream_t *stream, enum h2o_http
assert(conn->delayed_streams.recv_body_blocked.prev == &stream->link || !"stream is not registered to the recv_body list?");
break;
case H2O_HTTP3_SERVER_STREAM_STATE_CLOSE_WAIT: {
+ if (h2o_linklist_is_linked(&stream->link))
+ h2o_linklist_unlink(&stream->link);
pre_dispose_request(stream);
if (!in_generator) {
h2o_dispose_request(&stream->req);
