BUG/MINOR: do not close uninit FD in quic_test_socketops()

On startup, quic_test_socketops() is called to ensure that chosen
configuration option are compatible with UDP system stack. A dummy FD is
allocated to invoke various setsockopt() settings.

If no tests are required, FD is not allocated. In this case, close()
should not be close. This is mostly for better coding as this does not
cause any real issue for users.

This should fix github issue #2638.

No need to backport.

src/proto_quic.c

@@ -577,7 +577,8 @@ static int quic_test_socketopts(struct listener *l)
 #endif
 	}
 
-	close(fdtest);
+	if (fdtest >= 0)
+		close(fdtest);
 	return ERR_NONE;
 
  err: