softpipe: fix crash when accessing null colorbuffer

Fixes piglit fbo-missing-attachment-blit test.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73755

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/drivers/softpipe/sp_quad_blend.c

@@ -1232,7 +1232,10 @@ choose_blend_quad(struct quad_stage *qs,
             softpipe->blend->rt[0].colormask == 0xf &&
             softpipe->framebuffer.nr_cbufs == 1)
    {
-      if (!blend->rt[0].blend_enable) {
+      if (softpipe->framebuffer.cbufs[0] == NULL) {
+         qs->run = blend_noop;
+      }
+      else if (!blend->rt[0].blend_enable) {
          qs->run = single_output_color;
       }
       else if (blend->rt[0].rgb_src_factor == blend->rt[0].alpha_src_factor &&