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.