HTTP/3: kept the skipped frame length separate from data length

HTTP/3: kept the skipped frame length separate from data length

ngx_http_v3_parse_data() stored the remaining length of a skipped
(unknown) frame in st->length -- the same field the request body
filter uses for the number of DATA payload bytes to pass on as
request body.  When an unknown frame was split across parser
invocations, the skip was interrupted with st->length holding the
skip remainder; on the next buffer the body filter, which calls
the parser only while st->length == 0, treated that remainder as
request body.

When proxying, this could result in sending unframed bytes, potentially
allowing for bypass of client_max_body_size, which is enforced only on
the DATA path, or an injection.

The fix is to keep unknown frame payload in a separate st->skip counter.

Reported by Hansol Lee of GOTROOT.