anv/pipeline: Don't look at blend state unless we have an attachment

Without this, we may end up dereferencing blend before we check for
binding->index != UINT32_MAX.  However, Vulkan allows the blend state to
be NULL so long as you don't have any color attachments.  This fixes a
segfault when running The Talos Principal.

Fixes: 12f4e00b69
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/intel/vulkan/genX_pipeline.c

@@ -1351,10 +1351,10 @@ has_color_buffer_write_enabled(const struct anv_pipeline *pipeline,
       if (binding->set != ANV_DESCRIPTOR_SET_COLOR_ATTACHMENTS)
          continue;
 
-      const VkPipelineColorBlendAttachmentState *a =
-         &blend->pAttachments[binding->index];
+      if (binding->index == UINT32_MAX)
+         continue;
 
-      if (binding->index != UINT32_MAX && a->colorWriteMask != 0)
+      if (blend->pAttachments[binding->index].colorWriteMask != 0)
          return true;
    }