BUG/MEDIUM: h3: reject standalone FIN on invalid request/response

BUG/MEDIUM: h3: reject standalone FIN on invalid request/response

HTTP/3 parser has a special code path to handle a standalone FIN.
However for a request stream there was no check to ensure that a valid
request/response was received before reporting the closure.

This would result in an empty HTX buffer with EOM set. On the MUX level,
QCS instance would remain accounted in QCC <nb_hreq> counter, which
could result in unnecessary memory consumption.

Fix this rejecting a request stream closed with a standalone FIN without
a valid request or response received. The stream read is aborted and
write is reset using error code H3_REQUEST_INCOMPLETE or
H3_MESSAGE_ERROR depending on the proxy side. This allows to quickly
release the QCS instance.

This must be backported up to 2.6.