vnc: fix use-after-free in vnc_update_client_sync

Spotted by Coverity:

876     static int vnc_update_client_sync(VncState *vs, int has_dirty)
877     {

(1) Event freed_arg:    "vnc_update_client(VncState *, int)" frees "vs".  [details]
Also see events:        [deref_arg]

878         int ret = vnc_update_client(vs, has_dirty);

(2) Event deref_arg:    Calling "vnc_jobs_join(VncState *)" dereferences freed pointer "vs". [details]
Also see events:        [freed_arg]

879         vnc_jobs_join(vs);
880         return ret;
881     }

Remove vnc_update_client_sync wrapper, replace it with an additional
argument to vnc_update_client, so we can so the sync properly in
vnc_update_client (i.e. skip it in case of a client disconnect).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
