gtk: avoid oob array access

When too many consoles are created, vcs[] may be write out-of-bounds.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20161207105511.25173-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>