BUG/MINOR: mux-h1: don't report a 408 error on timeout of a closed connection

BUG/MINOR: mux-h1: don't report a 408 error on timeout of a closed connection

When a frontend connection times out, the mux H1 tries to send a 408 error
to the client unless the connection is in the CLOSING state. But a
connection may also be in the CLOSED state, waiting the client closes on its
side. In that case too, no 408 must be emitted. Because of this issue, the
failed requests counters are incremented, a "408 <BADREQ>" log line is
emitted while it should not.

Now the error is only sent if the connection is still alive.

It should be backported as far as 2.8.