check if available bytes is 0 before cleaning up
check if available bytes is 0 before cleaning up

there is a case where we have started streaming, received all data from the stream,
but have not processed it all before cleaning up, resulting in truncation and client hang-up.

Example: A POST request with 20kb of body will send the headers + H2O_HTTP3_REQUEST_BODY_MIN_BYTES_TO_BLOCK
and a Content-Length of 20kb, but the body will be smaller than this, resulting in the client waiting for more data

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