spirv: Assert that vtn_pointer_to_deref() doesn't return NULL

There are a few things that could cause us to return NULL here and we
should just fail to parse the SPIR-V if that ever happens instead of
crashing when someone tries to access it.

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36427>

src/compiler/spirv/vtn_variables.c

@@ -524,6 +524,8 @@ vtn_pointer_to_deref(struct vtn_builder *b, struct vtn_pointer *ptr)
       ptr = vtn_pointer_dereference(b, ptr, &chain);
    }
 
+   vtn_assert(ptr->deref != NULL);
+
    return ptr->deref;
 }