llvmpipe: fix crash when not all attachments are populated in a fb

llvmpipe: fix crash when not all attachments are populated in a fb

Framebuffers can have NULL attachments since a while. llvmpipe handled
that properly for lp_rast_shade_quads_mask but it seems the change didn't
make it to lp_rast_shade_tile.
This fixes piglit fbo-drawbuffers-none test (though I need to increase
the FB_SIZE from 32 to 256 so the tris cover some tiles fully).
https://bugs.freedesktop.org/show_bug.cgi?id=79421

Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>