BUG/MAJOR: mux-quic: fix BUG_ON on empty STREAM emission

BUG/MAJOR: mux-quic: fix BUG_ON on empty STREAM emission

A BUG_ON() is present in qcc_io_send() to ensure that encoded frame list
is empty if qcc_build_frms() previously returned 0.

This BUG_ON() may be triggered if empty STREAM frame is encoded for
standalone FIN. Indeed, qcc_build_frms() returns the sum of all STREAM
payload length. In case only empty STREAM frames are generated, return
value will be 0, despite new frames encoded and inserted into frame
list.

To fix this, change return value of qcs_send(). This now returns the
whole STREAM frame length, both header and payload included. This
ensures that qcc_build_frms() won't return a nul value if new frames are
encoded, even empty ones.

This must be backported up to 3.1.