[Mesa-stable] draw: fix segfaults with aaline and aapoint stages disabled

[Ian Romanick]

The following patch is marked for stable (I'm assuming just 9.2?), but
it does not apply cleanly.  Can someone fix it up and resent it to
mesa-stable?

Thanks.

commit 89a665eb5fa176f68223bf54a472d6a0567c3546
Author: Marek Olšák <marek.olsak at amd.com>
Date:   Tue Aug 27 21:57:41 2013 +0200

    draw: fix segfaults with aaline and aapoint stages disabled

    There are drivers not using these optional stages.

    Broken by a3ae5dc7dd5c2f8893f86a920247e690e550ebd4.

    Cc: mesa-stable at lists.freedesktop.org

    Reviewed-by: Jose Fonseca <jfonseca at vmware.com>