MINOR: mux-quic: refine reception of standalone STREAM FIN

MINOR: mux-quic: refine reception of standalone STREAM FIN

Reception of standalone STREAM FIN is a corner case, which may be
difficult to handle. In particular, care must be taken to ensure app_ops
rcv_buf() is always called to be notify about FIN, even if Rx buffer is
empty or full demux flag is set. If this is the case, it could prevent
closure of QCS Rx channel.

To ensure this, rcv_buf() was systematically called if FIN was received,
with or without data payload. This could called unnecessary invokation
when FIN is transmitted with data and full demux flag is set, or data
are received out-of-order.

This patches improve qcc_recv() by detecting explicitely a standalone
FIN case. Thus, rcv_buf() is only forcefully called in this case and if
all data were already previously received.