anv/descriptor_set: Fix binding partly undefined descriptor sets

Section 13.2.3. of the Vulkan spec requires that implementations be able to
bind sparsely-defined Descriptor Sets without any errors or exceptions.

When binding a descriptor set that contains a dynamic buffer binding/descriptor,
the driver attempts to dereference the descriptor's buffer_view field if it is
non-NULL. It currently segfaults on undefined descriptors as this field is never
zero-initialized. Zero undefined descriptors to avoid segfaulting. This
solution was suggested by Jason Ekstrand.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96850
Cc: 12.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/vulkan/anv_descriptor_set.c

@@ -409,6 +409,11 @@ anv_descriptor_set_create(struct anv_device *device,
       (struct anv_buffer_view *) &set->descriptors[layout->size];
    set->buffer_count = layout->buffer_count;
 
+   /* By defining the descriptors to be zero now, we can later verify that
+    * a descriptor has not been populated with user data.
+    */
+   memset(set->descriptors, 0, sizeof(struct anv_descriptor) * layout->size);
+
    /* Go through and fill out immutable samplers if we have any */
    struct anv_descriptor *desc = set->descriptors;
    for (uint32_t b = 0; b < layout->binding_count; b++) {