BUG/MEDIUM: quic: fix transient send error with listener socket

BUG/MEDIUM: quic: fix transient send error with listener socket

Transient send errors is handled differentely if using connection or
listener socket for QUIC transfers. In the first case, proper poller
subscription is used via fd_cant_send()/fd_want_send(). For the listener
socket case, error is ignored by qc_snd_buf() caller and retransmission
mechanism will allow to reemit the data.

For listener socket, transient error code handling is buggy. It blindly
uses fd_cand_send() with <qc.fd> member which is set to -1 for listener
socket usage. This results in an invalid fdtab access, with a possible
crash or a modification of a totally unrelated FD.

This bug is simply fixed by using qc_test_fd() before using
fd_cant_send()/fd_want_send(). This ensures <qc.fd> is used only if
initialized which is only the case when using connection socket.

No crash was reported yet for this bug. However, it is reproducible by
using ASAN compilation and the following strace sendmsg() errno command
injection :

 # strace -qq -yy -p $(pgrep haproxy) -f -e trace=%network \
     -e inject=sendto,sendmsg:error=EAGAIN:when=20+20

This must be backported up to 2.7.