vnc: make sure we finish disconnect

It may happen that vnc connections linger in disconnecting state forever
because VncState happens to be in a state where vnc_update_client()
exists early and never reaches the vnc_disconnect_finish() call at the
bottom of the function.  Fix that by doing an additinal check at the
start of the function.

https://bugzilla.redhat.com/show_bug.cgi?id=1352799

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1468405280-2571-1-git-send-email-kraxel@redhat.com
