anv: assert that the target bo is valid when adding a reloc list

this catches some undefined behavior like e.g., using a stale descriptorset
that references deleted bos, which I would absolutely never do

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6747>

src/intel/vulkan/anv_batch_chain.c

@@ -200,6 +200,8 @@ anv_reloc_list_add(struct anv_reloc_list *list,
    if (result != VK_SUCCESS)
       return result;
 
+   assert(target_bo->gem_handle > 0 && target_bo->refcount > 0);
+
    /* XXX: Can we use I915_EXEC_HANDLE_LUT? */
    index = list->num_relocs++;
    list->reloc_bos[index] = target_bo;