v3dv: do not access member of a NULL structure

Check if the structure is NULL before trying to get access to its
members.

This has been detected by the Undefined Behaviour Sanitizer (UBSan).

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29911>

src/broadcom/vulkan/v3dv_device.c

@@ -1655,7 +1655,7 @@ v3dv_GetInstanceProcAddr(VkInstance _instance,
                          const char *pName)
 {
    V3DV_FROM_HANDLE(v3dv_instance, instance, _instance);
-   return vk_instance_get_proc_addr(&instance->vk,
+   return vk_instance_get_proc_addr(instance ? &instance->vk : NULL,
                                     &v3dv_instance_entrypoints,
                                     pName);
 }