`http2client.c`: a 0-byte data frame is ok in message_body_forbidden cases.
`http2client.c`: a 0-byte data frame is ok in message_body_forbidden cases.

This makes the check introduced in h2o/h2o#2971 slightly less strict.

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