vnc: avoid possible file handler leak

vs->lsock may equal to 0, modify the check condition,
avoid possible vs->lsock leak.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -3644,7 +3644,7 @@ void vnc_display_open(const char *id, Error **errp)
             if (vs->ws_enabled) {
                 vs->lwebsock = inet_listen_opts(wsopts, 0, errp);
                 if (vs->lwebsock < 0) {
-                    if (vs->lsock) {
+                    if (vs->lsock != -1) {
                         close(vs->lsock);
                         vs->lsock = -1;
                     }