BUG/MINOR: quic: fix fd leak on startup check quic-conn owned socket

A startup check is done for first QUIC listener to detect if quic-conn
owned socket is supported by the system. This is done by creating a
dummy socket reusing the listener address. This socket must be closed as
soon as the check is done.

The socket condition is invalid as it excludes zero which is a valid
file-descriptor value. Fix this bug by adjusting this condition.

In theory, this bug could prevent the usage of quic-conn owned socket as
startup check would report a false error. Also, the file-descriptor
would leak as it is not closed. In practice, this cannot happen when
startup check is done after a 'quic4/quic6' listener is instantiated as
file-descriptor are allocated in ascending order by the system.

This should fix github issue #1954.

quic-conn owned socket implementation is scheduled for backport on 2.7.
This commit must be backported with it, more specifically to fix the
following patch :
  75839a44e7
  MINOR: quic: startup detect for quic-conn owned socket support

src/proto_quic.c

@@ -598,7 +598,7 @@ static int quic_test_sock_per_conn_support(struct listener *l)
 	if (ret) {
 		fdtest = socket(rx->proto->fam->sock_domain,
 		                rx->proto->sock_type, rx->proto->sock_prot);
-		if (fdtest > 0) {
+		if (fdtest >= 0) {
 			if (setsockopt(fdtest, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) &&
 			    bind(fdtest, (struct sockaddr *)&rx->addr, rx->proto->fam->sock_addrlen) < 0) {
 				ha_alert("Your platform does not seem to support multiple UDP sockets binded on the same address. "