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