vnc: fix segfault in vnc_display_pw_expire()

NULL pointer dereference in case no vnc server is configured.
Catch this and return -EINVAL like vnc_display_password() does.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
