anv: avoid segmentation fault due to vk_error()

anv: avoid segmentation fault due to vk_error()

vk_error() is a macro that calls __vk_errorf() with instance == NULL.

Then, __vk_errorf() passes a pointer to instance->debug_report_callbacks
to vk_debug_error(), which segfaults as this pointer is invalid but not
NULL.

Fixes: e5b1bd6ab8 "vulkan: move anv VK_EXT_debug_report implementation to common code."
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>