BUG/MINOR: mux-quic: disable fast-fwd if connection on error

BUG/MINOR: mux-quic: disable fast-fwd if connection on error

Add a check on nego_ff to ensure connection is not on error. If this is
the case, fast-forward is disable to prevent unnecessary sending. If
snd_buf is latter called, stconn will be notified of the error to
interrupt the stream.

This check is necessary to ensure snd_buf and nego_ff are consistent.
Note that previously, if fast-forward was conducted even on connection
error, no sending would occur as qcc_io_send() also check these flags.
However, there is a risk that stconn is never notified of the error
status, thus it is considered as a bug.

Its impact is minimal for now as fast-forward is disable by default on
QUIC. By fixing it, it should be possible to reactive it soon.

This should be backported up to 2.9.

(cherry picked from commit 19f4f4d890bc28bbe4b850e1ced9be399f59fafb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>