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>
