vnc: fix segmentation fault when invalid vnc parameters are specified

Reproducer:
 #./qemu-system-x86_64 -vnc :0,ip
qemu-system-x86_64: -vnc :1,ip: Invalid parameter 'ip'
Segmentation fault (core dumped)
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -3703,8 +3703,13 @@ QemuOpts *vnc_parse_func(const char *str)
 {
     QemuOptsList *olist = qemu_find_opts("vnc");
     QemuOpts *opts = qemu_opts_parse(olist, str, 1);
-    const char *id = qemu_opts_id(opts);
+    const char *id;
 
+    if (!opts) {
+        return NULL;
+    }
+
+    id = qemu_opts_id(opts);
     if (!id) {
         /* auto-assign id if not present */
         vnc_auto_assign_id(olist, opts);