MINOR: mux-quic: do not count stream flow-control if already closed

It is unnecessary to increase stream credit once its size is known.
Indeed, a peer cannot sent a greater offset than the value advertized.
Else, connection will be closed on STREAM reception with
FINAL_SIZE_ERROR.

This commit is a small optimization and may prevent the emission of
unneeded MAX_STREAM_DATA frames on some occasions.

It should be backported up to 2.7.
1 file changed