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

ui/vnc.c

@@ -2849,6 +2849,10 @@ int vnc_display_pw_expire(DisplayState *ds, time_t expires)
 {
     VncDisplay *vs = ds ? (VncDisplay *)ds->opaque : vnc_display;
 
+    if (!vs) {
+        return -EINVAL;
+    }
+
     vs->expires = expires;
     return 0;
 }