vnc: fix overflow in vnc_update_stats

Commit "bea60dd7 ui/vnc: fix potential memory corruption issues" is
incomplete.  vnc_update_stats must calculate width and height the same
way vnc_refresh_server_surface does it, to make sure we don't use width
and height values larger than the qemu vnc server can handle.

Commit "e22492d3 ui/vnc: disable adaptive update calculations if not
needed" masks the issue in the default configuration.  It triggers only
in case the "lossy" option is set to "on" (default is "off").

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1485248428-575-1-git-send-email-kraxel@redhat.com